Unrooting Verizion nexus 6 on 5.1.0 build number LMY47E

I have been rooted for a few months now and need to get a replacement phone from vzw. I have been searching for the better part of 2 days on how to completly unroot my nexus 6. i used the method used in "[HOW-TO] The Nexus 6 All-In-One Beginner's Guide By xBeerdroiDx" to root it. Which installed TWRP, with any other phone ive rooted i could just go to SuperSU and click full unroot and it would do it. That is not the case now. Can i get a step by step directions from someone on how to unroot my phone please.

Thanks in advance.