Disclaimer: I will not take responsibility for any bricked devices
This guide applies to owners of rooted nexus 5 devices that run stock version of android 4.4.4
As you might know by now, Google released android 5.0 (Lollipop) a while back along with its flagship device, Nexus 6. Different android devices makers have set different timelines for rolling out the new update. Previous Nexus devices owners are also in line to receive the update some time soon but Google is yet to give a specific date. Before then, nexus devices owners can download the update from this link and manually apply it.
If you're rooted and running a custom recovery, you need to revert to the stock recovery first. You can get that by extracting it from the image zip file in this link then flash it using fastboot "fastboot flash recovery filename.img"
You then install the update into your device using "adb sideload update.zip". The update should then install smoothly and your apps and appdata should all be retained.
Additional guides can be found here
All the best