mobile

Hi,so basically i attempted to root my phone,kinda decided not to after unlocking bootloader,so i locked it again,then when i try to download anything,it fails,regardless of wireless or mobile data,it wont sync with htc sync manager,just error 4, meaning i cant download or put music on my phone

So im wondering if someone can tell me how to set it back to factory rom,so it gets rid of the Super user installer,and locks the bootloader again and doesnt show its been tampered with
Ive looked at tutorials but none have worked so far

Phone details:

HTC One M8
02 network Contract
5.0.1 android version
4.20.206.1 - I cant remember what this number exactly was but something to do with the software version i believe