Note: by using this tutorial you accept responsibility for any damages that may occur to your device
Firstly, make sure you are using an supported W10M build
Read the 'Getting started' in wpinternals menu.
Connect your handset to PC and download stock ffu by tapping 'download all' in 'download' section
Note: some devices needs removal of sdcard for successful unlock
Tap on unlock bootloader and let it alone until it completes. The device will reboot many times , normal
Hold power button down when prompted to boot back into the os. You may also enable root access after rebooting back into os.
Once bootloader is unlocked you won't receive software updates. Have fun!😀