Windows 10 Anniversary update is out!! Well there is good news and bad news as well. Good News- Windows 10 Anniversary update is awesome and its stability is also good it is available for all mobile as well as for PCs. Bad news is that now old lumia(925,920,820,1020) will not even be able to install threshold build that means no Windows 10 update when you downgrade to WP8.1.
All the registry hack tools for WP8.1 require Memory card so folks running WP8.1 with no memory card are helpless but, but there is a way you can install Windows 10 Mobile. This process is tedious and requires lot of patience. Don't give up!!
Step 1- Install WDRT so that you can get all the necessary drivers. you can easily search for it on google.
Step 2- Install Windows Phone Internals from here. thanks to @Heathcliff74.
Step 3- Check the getting started page of WPInternal.
Step 4- Now first we will unlock our bootloader and to do that we will be using WPInternal.
Step 5- For Lumia 925 you have to select FFU file from C:\ProgramData\Microsoft\Packages\<Your Device Revision>\ Remember to select FFU file of the same version as of the version installed on your phone. If you have an FFU that is of older version than please flash it first with the FFU then proceed.
Step 4- Then download the Emergency Flash Loaders by going to http://lumiafirmware.com. I have included for Lumia 925. It should be a .HEX file.
Step 6- Then select the SBL3 engineering file and to enable ROOT access it is IMPORTANT to Select this file. I have included some files.
Step 7- Now click continue and wait for the process to complete.
Step 8- Now you can Enable root access.
Step 9- Open RegEdit in your PC and click HKEY_LOCAL_MACHINE and then select files == > Load Hive.
Step 10- Now in the dialog window navigate to MainOS ==> Windows/system32/config and select SYSTEM, it will ask for a name and name it such that you are not confused.
Step 11- Now change the necessary keys. In this you have to navigate to Platform-->DeviceTargetingInfo and then need to change
PhoneManufacturer = NOKIA,
PhoneManufacturerModelName = RM-984_1006,
PhoneModelName = Lumia 830,
PhoneHardwareVariant. = RM-984
For Lumia 925 I used the details of Lumia 830 Indian Variant. You can use the supported lumia which is close to your model in terms of spec.
and after editing click unload Hive this will save your data and everything
Step 12- Disable root access and press power+volumeDown key to soft reset and start your device.
Step 13- Download Windows Insider and select preferred ring and you should see an update. Note after various updates release preview ring doesn't work for Lumia 830, so if you are using Lumia 830 or any unsupported devices then please use slow or fast insider ring
I would also like to thanks to @tofuschnitte.