It depends on whether you made the contribution before the deadline (or extension deadline) for filing your 2020 taxes.
If you made the contributions for 2020 after you filed your 2020 tax return, you will include the amounts with your 2021 contributions on your 2021 tax return, but you do not have to separately state the amount.
If you deposited the 2020 contributions before the return filing deadline you would need to amend your 2020 tax return.
To enter your SEP contributions in TurboTax Online please see Where do I enter my SEP IRA contributions?
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