How do I go about unrooting a used T Mobile Nexus 6?

[AC Question]

Unrooting a used T Mobile Nexus 6

Hi. I hope I am in the right place. I havent ever dealt with Android mods before. I purchased a used T Mobile Nexus 6. After having problems getting wifi calling to work, I found that the phone is already rooted, with twrp 2.8.6. I want to unroot the phone, but I cant find the guy who rooted it. I'm not entirely sure about factory backup and all that stuff. How can I tell if the ROM on the phone is stock, or something like shamu? Is a factory restore safe? I just dont want to unroot it without knowing if it is safe.

 

[jdk2]

Does the phone still have his Google account on it? If so, you need him to remove it from the phone before you start making changes to it.