2019 or prior can be amended within 3 years of their due date including any valid extension. if you already submitted 2020
the return at this point can't be recalled. the IRS won't open e-filing until 2/12/2021. so you'll have to wait. if your return is rejected you can fix it and resubmit. if it is accepted you'll have to wait until it is fully processed and then file an amended return. it is possible the IRS will make changes to your return and adjust your refund accordingly so you won't have to amend. watch for a notice from the IRS telling you what to do.