I had to amend my 2020 taxes. After completing the amending process, I will receive a refund. How do I apply the refund to my estimated taxes?
Please see How do I apply my refund to next year's (2021) taxes?
Taxes are pay-as-you-go. This means that you need to pay most of your tax during the year, as you receive income, rather than paying at the end of the year, to avoid underpayment penalties.