The payment date for deferred taxes has been extended to January 3, 2022 for the first half and January 3, 2023 for the second half.
You are correct that you can't amend your current payment plan for income taxes; however, individuals who are unable to pay the full deferred tax amount should pay whatever they are able to pay by the installment due dates to limit penalty and interest charges.
If the installment amount is not paid in full, IRS will send the taxpayer a balance due notice. Taxpayers should follow instructions on the notice to make a payment or apply for a payment plan. They can also visit the Paying Your Taxes page on IRS.gov for additional information about ways they can pay, what to do when they can't pay, and viewing their tax account.
Click this link for more detailed info on Deferred SE Tax Payments.