Typically , the federal AGI should remain consistent with what was reported on your Form 1040. If there were no changes to your 1040 and you are just amending the state return for a state adjustment, then your federal AGI would remain the same.
If you still find it necessary to change, your only option to do so would probably be with TurboTax Desktop. You would have to manually override the amount in Forms mode and mail in the amended return. To switch to TurboTax Desktop, click here. For full instructions on overriding amounts in TurboTax Desktop, click here.