If you haven't filed your return then please follow these steps to make changes to the stimulus amount:
If your return has been accepted you will have to wait and see if you receive your refund that includes the correct credit. If not then you will need to amend your return (1040X) and send the funds back to the IRS. Please see FAQ How to amend a tax return for instructions.
Please see IRS Recovery Rebate Credit Q D2 What happens if I claim the incorrect amount? for additional information.
Please see the links below when Form 1040X is available: