Htc one m9 unable to find firmware files for update from Android 5 to 6
You need to check wich firmware version your rom needs. For latest viperonem9 it's 3.35.401.12.

Now you can go to the xda-developer page http://forum.xda-developers.com/one-m9/development/firmware-t3068243 to find the latest htc firmwares for the m9 with a detailed guide.

Be aware that the fullstock firmwares force a full wipe, so you will lose your data. To avoid that you can use the nowipe firmwares wich are linked on the same page.