well there is another way but the process is a bit long? go to sammobile.com go the te selection that say firmware search for your model number wait for compleet and follow the steps that will tell u how to use Odin to update your phone.
@Dinu Just to clarify are you using sprint S20 branded phoen and it is unlocked and now youa re using at&t sim card? Did the phone reboot to at&t firmware? if so you should be able to update the phone.