My nexus 7 was showing "update available" but after thinking "You know if it's crap I don't want to deal with it now" (and reading later that there were some problems) I left it, 2 weeks later (a week ago) it came back, and I ignored it.
Now I want to start using Material Design, but the update has gone, have I missed my window? I doubt it but Google's update policy...
Anyway, can I "force" an update?
fastboot
tool (which is part of the Android SDK or as a minimal package) on a computer and a USB connection. Detailed instructions are on the images site.