HTC 820 stuck in bootloop after update 6.0

i have HTC 820 dual sim D820u India
i upgraded my mobile to Android 6.0. then it suddenly switched off when i tried to click a pic with the camera.
now it is stuck in bootloop.
i can access recovery.
but whenever i select and press factory reset, and recovery it gets restart. and again stuck in bootloop. im not able to do factory reset.
please tell me what to do.
mobile was unrooted.

Sorry for bad English.