Returning HTC one to stock? (newbie)

[AC Question]

Hi

So recently I bought a second hand HTC one from an exchange store and I have been trying to update to the latest software/sense version and can't do it. I recently found out when restoring the phone that it has been rooted and says ***TAMPERED*** at the top of the boot loader.

I personally don't like my phones being rooted and would like to return it to factory, out of the box settings with the original stock rom that it should have. There is something called Team Win on the phone that I want removed and their website is unhelpful in telling me how to do that.

I am a complete newbie at this. I never root my phones and even though I have looked through all of the tutorials to return the phone to stock rom I don't understand half of it an I am worried I will ruin my phone.

My HTC one is from the UK, it is unlocked to any network. It is currently running Android 4.3, Sense 5.5 and is on software version 3.62.401.1.

I really need someone to run through with me exactly what to do to remove Team Win and exactly what downloads I need for this model.

Thank you.