For folks who don't want to wait for the Android 5.0 Lollipop OTA update to push out to their older Nexus devices, Google has made the factory images available for several of them. Of course, the other great reason to have the factory images is if you need to recover your device back to factory after any disastrous tinkering.
Here are those Android 5.0 Lollipop download links:
Here's the Android Quick Start Guide for you to get started flashing these: Android Quick Start Guide Android 5.0 Lollipop - Books on Google Play
Don't forget that everything on your device will be wiped if you flash this, so keep that in mind.