That makes no sense. Apple’s 14 day policy allows you to return the iPhone for a full refund if its been less than 14 days since you bought it. But that would only apply if you requested the 128GB version or did not specify the storage and they just gave you the 128GB version. You can get a full refund and then buy the version you want.
But if you specifically stated you wanted the 256GB version, and they gave you the 128GB version they are in error and should replace it. More so if you paid for the 256GB version since it is more expensive than the 128GB.
Go back to AT&T and tell them if its their mistake and you are paying for the larger capacity version that you want the larger capacity version.
If you agreed to the lower capacity version or did not specify the storage and you are within 14 days since you bought it, you can again return it for a full refund and buy the larger capacity version.