How to update my OS?

[AC Question]

I bought an s4 from AT&T and unlocked it to use on cricket network. How do I update the os because it's saying I don't have an update? Would I have to put in an AT&T SIM card?

 

[SpookDroid]

IF an update is available, it should show up in Kies (connect the phone to the PC via USB and fire up the software).

However, my friend has an unlocked Galaxy S3 from ATT, using it on a Mexican carrier and guess what? Even though I KNOW there's an update available for this phone, we've never been able to update it. No OTA, no Kies, nada. Don't know if you need to have the original carrier's SIM card to update, but it shouldn't.