@rbadis
I've had this phone before,but,it has been a while,two years & although the One series shares a lot of commonality as for methods of modifying/etc...,some things do change from model to model/year to year.
Therefore,I'm just gonna list some links for what appears to be a Sprint HTC M8 & have those better versed w/this particular version of the M8 advise.
Either way,there's no question your phone has been modified,an OTA isn't in the cards,you'll need to get it by other means.
http://androidforums.com/forums/root-htc-one-m8.2602/
http://forum.xda-developers.com/sprint-htc-one-m8/development
As Hadron mentioned,it may be necessary to restore some items to stock before updating,whether by a custom ROM or an HTC RUU.
Try the 1st link I posted,it's for the root section of the HTC One M8 forum right here @ A/F.
Not only some of the best expertise in all things HTC in that section,but,also,some of the nicest/coolest peeps you'll find anywhere in such forums.
If you're not familiar w/the topics here (modifying/root/ROMS/RUU's),that's
OK,we're all here to help one another & we'll do our best to get you on your way to getting that update.