Many of us were looking for a way to Upgrade HT M8 for Windows to W10P. Here are the steps I found in the internet, I have tested and it worked.
Thanks to @Bladeni...
Instructions to root HTC One M8 with Windows Phone 8.1 to install Windows 10 mobile 20 August 2016
1. Backup all your pictures, videos, music and files to your SD card or your Computer
2. Download the latest Windows Device Recovery Tool
3. Ensure that your battery charge is 40%.
4. Restart your phone and hold down the volume keys at the same time.
5. If there is a gear and lighting symbol restart the phone again because you were unsuccessful.
6. To restart the phone hold down the Power and volume down key until the phone vibrates
7. Immediately hold down the both volume keys.
8. One successful you will see a white screen appear with the phone serial number another information.
9. Connect to the phone with the USB cable to the computer with windows device recovery tool open
10. If the windows device recovery tool shows the message: My device was not detected
11. Click My device was not detected and click Other OEMs and Select HTC
12. Click next and continue without submission. Please note formatting your phone back to factory is very important to registering your phone in further steps.
13. At this point your phone should be formatting to Windows Phone 8.1
14. Complete initial setup of the phone on Windows Phone 8.1
15. Ensure that you signed in with a Microsoft account.
16. Download files WP8.0 SDK Tools Lite Setup Av1.20
17. Download files WP8.1 SDK Tools Lite Setup Av1.20
18. The files are normally in a zip format use WinRAR to extract them
19. Extract contents of the zip files and run the InstallWP80SDKToolsLite bat file.
20. Extract contents of the zip files and run the InstallWP81SDKToolsLite bat file.
21. Click start navigate to Windows phone SDK 8.0 then click Windows Phone developer registration.
22. Ensure that the device is on, unlocked and in the start screen.
23. Click Register. Wait, for the pop up window for you to enter your Microsoft account.
24. Enter to same Microsoft account that you are using on the phone.
25. One successfully completed you would see the option to Unregister.
26. Download the XAP file: CustomWPSystem_0500.xap
27. Note if the file is downloaded with a zip extension change the extension to XAP.
28. For example: CustomWPSystem_0500.zip to CustomWPSystem_0500.xap
29. Click start navigate to Windows phone SDK 8.0 then click application deployment.
30. Ensure that the device is on, unlocked and in the start screen.
31. Click browse in application deployment, navigate to the CustomWPSystem_0500.xap you downloaded
32. Select the file and click deploy.
33. Download the file CustomPDF_0003.xap as stated before if it is zip change it to xap
34. Copy CustomPDF_0003.xap to your phone internal memory in documents
35. Go to storage sense and set app, to be installed on SD card.
36. Open this website :
38. Use the above website as a visual reference
39. Open the deployed app CustomWPSystem on your phone
40. Follow the instructions in the app. Tap SD card permission
41. Select the SD card root folder this means navigate to the SD card and no other sub folder and tap the tick
42. SD card permission should be green
43. Click download from the store to download Preview for developers after the app is installed hold the back and app switch back to CustomWPSystem app
44. Scroll down and click select the XAP, navigate to the CustomPDF_0003.xap file located on your phone
45. Click Apply the Xap once finished
46. Open the Preview for developers app the icon has change to greenish color with a key
47. Follow these instructions
48. Please take screen shots by pressing volume Up and the power button at the same time before making changes
50. Use the above website as a visual reference
51. Open "HKEY_LOCAL_MACHINE" Or HKLM
52. Open "SYSTEM"
53. Open "Platform"
54. Open "DeviceTargetingInfo"
55. Once the Application was applied properly you will notice on the top a message: you can write hear
56. Open "PhoneManufacturer"
57. Ensure you take a screen shoot and note the original values
58. Click the Plus button at the bottom
59. In the first field enter: PhoneManufacturer
60. Change Type to String
61. In the last "Value Data" field, enter "NOKIA" (all caps). Click "Write" please do not enter inverted comma.
62. Press the back arrow in Windows phone 8.1
63. Open "PhoneManufacturerModelName"
64. Ensure you take a screen shoot and note the original values
65. Click the Plus button at the bottom
66. In the first field enter: PhoneManufacturerModelName
67. Change Type to String
68. In the "Value Data" field, enter "RM-1089_1087". Click "Write" please do not enter inverted comma.
69. Press the back arrow in Windows phone 8.1
70. Open "PhoneModelName"
71. Ensure you take a screen shoot and note the original values
72. Click the Plus button at the bottom
73. In the first field enter: PhoneModelName
74. Change Type to String
75. In the "Value Data" field, enter "Lumia 735". Click "Write"
76. Press the back arrow in Windows phone 8.1
77. Please note HTC sense tv can only be downloaded in Windows phone 8.1 store if you update before downloading it you will never be able to download for deploy it to your windows 10 mobile phone.
78. Once you have modified the registry
79. Download and open the Windows Insider app
80. Join the fast ring. Reboot your device when prompted.
81. After the reboot, check for updates and you should start downloading the latest Redstone build.
82. Repeat steps after windows 10 installation to restore your previous registry values
83. Do this only after you have reached the lastest build
84. The lastest build today in Redstone 2 is 14926
85. If you choose to update to this build be aware with that you must first remove your Security Pin
86. Your Sim Card will not work along with no keyboard
87. However there is hope once you factory reset your phone the 14926 build works great with none of the above in issues.
88. Please note however you will lose HTC sense tv app after the factory reset
89. To factory reset your phone goto Settings – About scroll to the bottom and click Reset you phone
90. You can choose release ring or Slow ring after build: 14393.187 or 14393.189
91. Do not update to build if you want HTC sense tv app
92. Download : InteropToolsApp_0.0.2.0_arm.appxbundle
93. Open Interop Tools locate in (Settings->Extras)
94. Click the "Registry Browser" button
95. Open "HKEY_LOCAL_MACHINE"
96. Open "SYSTEM"
97. Open "Platform"
98. Open "DeviceTargetingInfo"
99. Open "PhoneManufacturer"
100. In the "Value Data" field, enter "HTC" (all caps). Click "Write"
101. Open "PhoneManufacturerModelName"
102. Enter the appropriate Original Value below.
103. In the "Value Data" field, enter "HTC6995LVW". Click "Write"
104. Open "PhoneModelName"
105. In the "Value Data" field, enter "HTC6995LVW". Click "Write"