A Way To Get It Early
We mentioned AOSP and files on Google's servers. While Google does roll out its Nexus updates Over-the-Air, it also offers users a way to skip the OTA and download the update early. We expect Google to give Nexus 5 users this option here with Android 6.0.
This is the beauty of the Nexus program. Once the Android 6.0 update is released on AOSP, Nexus 5 users with unlocked bootloaders will be able to download and install the update skipping the long wait for the update OTA.
If you're not comfortable doing that, you'll also be able to sideload the OTA file once its discovered lurking on Google's servers. If you're not familiar with the process, now would be a great time to get caught up on how it works.