Help HTC One M7 stuck in failing update loop

What does it look like when the phone does the "processing" after rebooting? Just wondering whether it has been modified in any way that might interfere with the update. If you see an image of a little green android lying on its back that sounds normal. If you see a screen with "ClockworkMod" or "TWRP" on it then that will be the problem. If something else, can you describe it?

Otherwise, what I would do is boot into the bootloader (reboot the phone while pressing the "volume down" key). Let us know what it says at the top of that screen (especially locked/unlocked/modified, S-On/S-Off). Then select "recovery" and press the power key (or, if you can't see "recovery" select "fastboot" to bring up the other menu and then select "recovery"). If you have the stock recovery I expect you'll get a black screen with a red triangle, or maybe an android with a red exclamation mark. Then press and hold the volume up key then while holding it press the power key again. That will give you the recovery menu(*). From the menu select "wipe cache partition", then once that's done reboot it. Hopefully that will clear up a bad download and it will work next time, though it's only guesswork.

(*) Getting the menu is much simpler with a custom recovery like TWRP, but official updates won't work with those, which is why I said that if you have TWRP or CWM that's the problem. In that case, let us know, and also what the top of the bootloader screen says, and we can tell you what your options are then.

Ok, looks like a normal update install and not a custom recovery (as far as I remember the stock update - I've been using custom software for so long).

Nothing I described would cause a factory reset. To do that you'd have to select that option from recovery, or unlock the bootloader (which requires using a computer and cannot be done accidentally).

You can use RUU . but you have to find the correct RUU file