I just got a Verizon HTC one M8 from Craigslist. The guy told me it is rooted. I want to go back to stock, just like it came from the box. When I hold down the volume down and power buttons to get into the booltoader, it tells me at the top that it is locked and s-on. What do I need to do to return to stock. Superuser is installed and when I use the unroot within the Superuser app, it will unroot but return to rooted after a restart. Thank you.