If the box is incorrectly marked, amend return so box is not marked with explanation why it was checked on original return. If original return had full standard deduction allowed there should not be a change to refund/tax due. You will need to mail the amended return...….IRS is backlogged with unopened mail. Could be a long long time before the amended return hits the system. May be just as well off to wait until the 2020 tax return is filed and get the $1200 credit.