I have exhausted what I can find perusing the internet so I figured I'd reach out here to see if anyone can help me. The download on the site is for Marshmallow and even though I have Lollipop I figured I'd give it a shot. I bought this nexus with nethunter already installed and, though it works for the most part, it is old and I can't seem to update it with sudo apt update and sudo apt upgrade. It can't find the repositories that are in sources.list. Does NetHunter use the same repos as Kali? I would imagine not but I'm not sure.
Basically my question is twofold:
1. Should I try changing my repos first and see if that works and, if so, which are the best to add to my list?
2. Which image would work for me to install if I need to reinstall NetHunter to get my Nexus as up-to-speed as possible?

Model: Nexus 7 (wifi)
Android Version: 5.1.1
Kernel Version: 3.1.10-Bricked-v0.591-g0a2e5f6
Build #: LMY47V

I understand it is an older tablet and my options may be limited but I'd like to get it as functional as possible for now. Thanks and any help is greatly appreciated!