According to AT&T’s support page, the update clocks in at 46.7MB large. After you snag the download and install the update, you should be on software version number H81010l and Android 5.1 build LMY47D. The update should be pushing out now, but it may take a day or two to show up for your device. If you’d like to check manually, you can do so from your phone’s settings menu.
Thanks to Trutz for the tip!