Flash upon the android one 5.1.1 you are already using from hovatek using download only in sp flash tool and also u can deselect userdata, cache and even system for you not to loose your data or any customize thing in the phone memory....
If you see the yellow triangle that means that your
Phone can boot into Recovery, all you needed to do
Is this: (once you see the yellow triangle) press down
The power button for 2secs and then tap the volume+
Button once and you will go into recovery.
2) if you don't want to be seeing that yellow triangle,
The flash my AndroidOne Rom I just gave you the link
With SP flash tool, this time select firmware upgrade
But know that it will wipe every data in your phone memory
Everything in the external Sdcard is safe
NOTE,
Vote of thanks to Bolumstar at XDA-Developers forum