Confirm the iPhone is eligible for unlock
Pay off any remaining device balance
Complete any required AT&T service commitment
Make sure the account is in good standing
Get the iPhone IMEI number
Submit an unlock request on the AT&T Device Unlock Portal
Wait for AT&T approval
Follow the unlock confirmation instructions from AT&T
Insert a SIM card from another carrier
Restart the iPhone if needed
If the device is still locked, contact AT&T support