You can qualify for a plus up payment if the IRS used your 2019 tax information for the third stimulus payment but your 2020 tax situation changed. Examples are: If you had a child in 2020 or your adjusted gross income (AGI) from 2019 disqualified you but your AGI in 2020 is under the limit.
If you qualify there is nothing you need to do. The IRS will issue the plus up payment after they processed your 2020 return. If the IRS has your direct deposit information they will send it to that account otherwise you will receive a check.