My federal return is accepted and complete but now the refund amount has changed how can I fix?

If your refund is less the amount of your Recovery Rebate Credit (shown on Line 30 of your 1040), it is very likely that the IRS adjusted your refund to decrease or eliminate your Recovery Rebate Credit. 

If you did not report all of your stimulus payments received and then received the Recovery Rebate Credit on your tax return, the IRS will adjust your refund to reflect the stimulus payments you actually received and were qualified for.

Per the IRS, If you entered an amount on line 30 but made a mistake in calculating the amount, the IRS will calculate the correct amount of the Recovery Rebate Credit, make the correction to your tax return and continue processing your return. If a correction is needed, there may be a slight delay in processing your return and the IRS will send you a notice explaining any change made.

2020 Recovery Rebate Credit — Topic G: Correcting issues after the 2020 tax return is filed