- First of all, make sure your Bootloader is unlocked. Check back with your device manufacturer on how to do that.
- Make sure you have adb and fastboot installed.
- Download the latest GSI and unpack it if needed.
fastboot flash system image.img(replace image.img with the proper image you’d like to flash)
- Wipe the cache and userdata partitions using stock recovery when possible
- Make sure you set up your device as ‘new device’ without restoring backups as you will run into problems if you do.