Google today posted updated Android 5.1 factory images for the Nexus 5 and Nexus 6. The new factory images are still based on Android 5.1, but carry a newer build number: LMY74I.
The new build of Android 5.1 mainly contains some fixes related to SIM cards, and does not contain any device specific changes. The company has also pushed LMY74I build of Android 5.1 to AOSP so that developers can play around with it as well.
Google had started rolling out the LMY74I OTA update to Nexus 6 owners last week itself. If you are yet to receive the update, you can always manually flash the factory image, though you will lose all your installed apps and their data.
You can download the new LMY74I factory images for here. Updated factory images for Nexus 4, Nexus 7 and the Nexus 9 are still not available for download.