The latest update was to 6.0.1 (the April 2017 update - QD1). The March file (QC1), the latest one I could find, is at
https://androidfilehost.com/?fid=745425885120723808. (Whether that's the update or the full firmware I'm not sure, but at 2GB it's probably the full firmware. Just load it into Odin (the AP slot if it's a single file) and flash it. If AT&T still has the April update out, you should get it in a week or so at most. (Or once you've flashed it, you could go to Settings and try to force an update to QD1 (in About Phone, if I remember Marshmallow.)