Unrooting a phone with no backup and no idea! Can I get some help?

[AC Question]

Unrooting a phone with no backup and no idea!

I'm new here and I'm almost certain this question has been asked many a time before but, I have googled and googled and googled and seem to get little snippets from here there and everywhere but not quite the full answer a newbie needs.

So, the issue is that I bought my HTC One M8 from ebay last year. I'm pretty certain it is rooted and I am trying to take it back to stock version so I can update it to the latest software and because I have no idea about flashing new firmware etc.

Clearly I do not have any form of backup as I did not unroot the phone originally, what makes it even more difficult is that I am unaware which carrier it originally was with (I am with 02). So is there a way I can go about unrooting it still? And, if so, can somebody please let me know what specifically what I need to download and the process by which I do it.

I'm sorry to be so difficult, but I really want to unroot this and just cannot seem to work out how to do it or, if I can even do it!

Any help very much appreciated.