Latest Samsung Galaxy S5 (SM-G900A) - ATT firmware
Samsung Galaxy S5 (SM-G900A) firmware for USA (AT&T) (ATT): 2 builds in the SamMobile archive, newest first. The latest is G900AUCS4DQD1 (Android 6.0.1, built 11 April 2017). Open a build to download it, or switch region with the country selector.