1. Unlock your bootloader (https://blog.flexispy.com/unlock-bootloader-htc-phone/)
2. Flash TWRP (https://www.xda-developers.com/how-to-install-twrp/) (https://www.androidfilehost.com/?w=files&flid=123824&sort_by=date&sort_dir=ASC)
3. Flash SuperSU with TWRP...