Google Nexus 7 — 2012 nexus 7 won't update andriod past 4.2.1

I have a 2012 nexus 7 that I rarely use. I went to install new microsoft word app that requires at least 4.4. My tablet has 4.2.1 and works perfectly but it has never updated. When I try to update I get the little green droid lying on his back with the Orange triangle with exclamation point. Then it goes back to working fine with 4.2.1. I tried factory reset but nothing changed. Still running 4.2.1

Thanks

T