Update to Android 5.0 Lollipop on Nexus 7
First of all, you get updates to your device from google on the basis of the date on which you bought the device. That is if you bought a tab 5 months ago and one yesterday the one which was bought earlier will receive update first. So don't worry when you have upgrade chance you will be notified.

Did your tab notified you that you have a upgrade available?
If yes did you actually download the upgrade for 5.0?