upgrade to lollipop 5.1 using twrp?
I tried to flash the rom manually but failed miserably (ran into this page).

I decided to use the Nexus Root Toolkit (download here) to go back to stock Android 5.1 (losing everything on the device of course). That worked like a charm and re-rooting it afterwards was simple enough Rooting guide for 5.1.1.

In the past whenever a new Android version came out I had to flash back to stock Android to get the update and then re-root it. I'm sure some people manage to update without erasing everything...