How do I downgrade from Lollipop back to KitKat on htc desire 816 How to go back to android kitkat from lollipop in htc desire 816 dual sim?

factory data reset and phone reset in settings, i have tried.
You would want to look for official stock firmware(Kitkat) provided by HTC, if you find one, you can flash it and get back to Kitkat. At least that's what I did on my Nexus 5.