Google and Samsung's Galaxy Nexus has long been out of the loop when it comes to official software updates, but that isn't stopping some users from getting to sink their teeth into Lollipop as the team of developers who put out CyanogenMod, have just pushed a new nightly recently that brings the Galaxy Nexus forward to Android 5.1 Lollipop. The nightly is a release of CyanogenMod 12.1 and is for the GSM model of the Galaxy Nexus only, so for users who may still be working with the Verizon CDMA model, it's worth pointing out that this build of the Lollipop ROM will not work on your device.
For any GSM users, all one needs to do is first make sure that the bootloader is unlocked and all the necessary steps have been taken to root your phone. This process has to be done before you can flash any custom firmware to the device which is likely information users may already have if they're aware of CyanogenMod in the first place. The Galaxy Nexus may be an older device, but the development community working with CyanogenMod wasn't ready to let it go just yet. The CyanogenMod 12.1 nightly was released just today, and there's no clear indication of how many nightlies the CyanogenMod team will keep pushing out to the phone before they decide to stop third-party development for it.
The download for the nightly file is about 248MB, so it's not a gigantic download but it's hefty enough that you'll probably want to stay connected to WiFi for the process as it should go quicker and it won't eat at your data. In addition to the nightly file you'll also need to make sure and pick up the correct Gapps package and flash that as well to get things like Gmail, the Play Store and other Google apps on the phone. Whether or not you've flashed ROMs before, it's worth noting that nightly builds are not considered completely stable so users who choose to install them may come up against bugs that need fixing. This is the nature of nightly builds and if developers continue work for the Galaxy Nexus with CyanogenMod 12.1, a new build attempting to fix any issues that may arise could show up as early as tomorrow.