Depend on several things OTA updates fails on your rooted devices or you want to manually upgrade your android.
Download the factory image of your Nexus from:
Extract the archive and edit the flash-all file (.bat if you are under Windows or .sh if you are under Linux).
You have to remove the “-w” option from the command on line 23:
fastboot -w update image-....
It will become:
fastboot update image-.....
Reboot in fastboot mode with:
adb reboot bootloader
And then you can run the flash-all script.