Do not do it if you do not have a clear vision of what you are doing: for inexperienced users it is a way to get a "paperweight", I do not accept any responsibility: you do it at your own risk!
This example refers to the European version of the Oreo firmware already in use: 2.33.401.10 NO BRAND(it can also be used with other versions)
• HTC U11 smartphone NO BRAND version with Android Oreo vers, 2.xx.xxx.xx Dual Sim NO BRAND;
• Bootloader unlocked;
• TWRP Recovery installed;
• Status: S-ON (S-OFF as you know is not permanent).
• European version of the RUU Nougat 1.27.401.11;
• Minimal ADB / Fastboot;
• TWRP Recovery (latest version);
• TOOL Htc_Version_Tool.zip thanks @sabpprook for the great tool
PREPARATION FOR THE DOWNGRADE via PC WINDOWS 10 and earlier
• Download the RUU on your PC and rename it to 2PZCIMG (leave the .zip extension untouched) and copy it to
the external SD card of your mobile phone;
• Install on the PC Minimal ADB preferably renaming the destination in C: \ ADB for convenience;
• Download the TWRP and copy it into the c: \ ADB folder renaming it to twrp.img for convenience;
• Download HTC_Version_Tool and unzip it to a folder on your PC as you wish.
INSTALLATION FOR THE DOWNGRADE IN ANDROID NOUGAT
• Restart the phone in Download Mode (press the LOW VOLUME + POWER BUTTON simultaneously) and connect
it to the PC using the supplied original USB cable;
• Run the Minimal ADB and Fastboot program on your PC (a DOS window will open with the c: \ adb prompt) and
leave it open;
• If you have not yet updated the TWRP Recovery run this command otherwise go to the next step:o fastboot flash recovery twrp.img
• Now you have to enter TWRP Recovery mode;
• Once entered, always with the phone left connected to the PC, type the command adb devices to check if our
cellphone has been recognized;
• Now run the HTC_Version_Tool program from the PC in the folder you previously unzipped and you will be asked to change the version number of your firmware:
HTC Version Tool, code by sabpprook
-- misc: /dev/block/sde1
-- offset: 2208
-- version: 2.33.401.10
new version: (type here the version you want to install as an upgrade: I recommend the latest release of Nougat)
confirm with the ENTER key (if you have done something wrong just repeat the procedure).
• You have now finished and you just have to restart it in download mode like this:
run, from your PC, the Minimal ADB and Fastboot program and, at the prompt, type adb devices to make it recognize and then adb reboot download and all that remains is to start the RUU flash previously copied to the External SD card.
For now we do not have S-OFF, this can be useful for flashing the RUU to remove problems and repair the phone when there is not a current RUU running.