Just like other HTC’s devices, the ROM Update Utility (RUU) could also be offered for all HTC One variants around the globe (AT&T, Sprint, T-Mobile, Rogers, O2, etc.). This utility is a Windows executable (.exe) file that contains the stock firmware, radio and hboot. This Utility is also uniquely available for each device models, which means you can’t run the T-Mobile’s RUU.exe over the AT&T device, the installation process will be aborted due to CID information doesn’t match between phone and the RUU, anyway crossflash this utility is a bad idea.
The RUU is usually used as a last option for those who have problems with their devices such as stuck on bootscreen, bootloops or soft-bricked. However some users even use this utility to restore their device to the official unrooted ROM stock so that they can sell or return the device to the HTC for warranty claims. You need to remember that the RUU is not for devices with unlocked bootloader, you have to relock your device prior to running the ROM Update Utility for HTC One (M7).
As said before each device model has a different RUU file, so make sure you get the right RUU file for your HTC One.
Here you can get RUU file for some variants, such as the International version, U.S. (AT&T,Sprint,T-Mobile), Canada (Rogers, Bell), DE (O2,Vodafone,T-Mobile), UK (O2,Three), AU (Telstra,Optus). If the RUU is not yet available for your specific HTC One variant, you can use CWM/TWRP Nandroid Backups, in case you only want to restore to stock.
Thanks to BugsyLawson from XDA for the links. Please note we’ve not had time to test all the files above, so try at your own risk.