unroot an return phone to out the box state topic

hi guys/girls , back again as still not any further along with getting my phone back to stock condition ... it was rooted by a friend and dnt think it was done correctly , now I cannot get superSU OFF MY HTC ONE , I am in the UK and my carrier is o2 ...I want to unroot an relock bootloader can someone please help me as I need to take phone in for repairs .... any help would be very much appreciated