Before you proceed, make sure that your HTC One M9 isn’t rooted because it won’t work on rooted device.
sure that you’ve fully backed up your device, and this guide is
strictly used on T-Mobile HTC One M9 device, any other device outside
that would definitely brick the device.
1. Download the OTA Update (Android 5.1 lollipop) zip file from here, and move the zip file to your desktop.
2. After that, Enable USB debugging on your HTC One M9, simply go to settings >> Developers option
and enable from there. If developers option isn’t visible on your
device, go back to settings >> about phone >> build number
and tap build number several times (up to 7times) till it displays that
you’re now a developer.
connect your HTC One M9 device to your PC via USB cable and move the
downloaded file to the root of your device. Don’t move it to any folder.
you’ve moved the zip file to the root of your device, then disconnect
your device from your PC and boot to recovery menu. To boot to recovery
menu, simply press and hold Volume up + Volume down + Power button
together and this would boot your device to bootloader mode, then select
‘boot to recovery mode’ option with the help of power button.
5. After that, select “apply from phone storage” option from the recovery menu, and browse the zip file that you moved to your device.
6. Once you’ve selected the zip file, then installation of the update would begin.
7. Once the installation process is completed, then go back to recovery menu and reboot system.
That’s it. Your device would be rebooted, and you would begin to
experience Android 5.1 Lollipop on your T-Mobile HTC One M9. To verify
the update, simply go to Settings >> about phone >> Software information. You would see 2.7.531.6 and android version 5.1.