You need to restore a stock TWRP backup (stock ROM) and stock recovery in order to OTA update. See the following thread for the required files and guide: https://forum.xda-developers.com/htc...-twrp-t3086860
Alternately, if there is an RUU for your M8 version, you can use it to update. But not all M8 versions have RUU. Do fastboot getvar all, and post the results (delete IMEI and serial number before posting) if you want further advice.
EDIT: I see from your post history, that you asked this same exact question last year (and again the year before that), and I gave you the proper answer. Not sure why you feel the need to ask the same question over and over, when you've already been provided the correct answer.
If you don't understand the process, or have any specific questions about the process, then ask them. Otherwise, just asking the same question (how to return to stock for update) is just annoying.