The
AT&T model of the Galaxy S5,
G900A, does NOT run stock Samsung firmware but rather, is a hybrid carrier version. See
#1.6 of
40 Galaxy S5 models - Dummies Guide
Therefore, the AT&T firmwares are not available from
SamMobile or
Samsung Live, (with the exception of NCE, Android 4.4.2), and is not supported by the Samsung Kies PC Suite and unlike stock Samsung firmwares, if you switch carrier you will not receive the OTA update.
Your best bet is xda developers who can offer a .zip file of Android 5.0 that you can sideload. This is for competent users. Please read the following thread, carefully and thoroughly before proceeding...
[How-To] Sideload the 5.0 OTA