I forgot that I made an estimated tax payment this year, and I already filed my return. Now what do I do?

Don’t worry! If you suddenly remember a tax payment you made after you already filed your return, you can submit an amended return by filing Form 1040X. On Form 1040X, you can report the payment and request a refund.

Go to this section in Credit Karma Tax: Start Amended Return

Credit Karma Tax can help you file Form 1040X. When you submit your 1040X, you’ll need to include the original tax return and tax forms you’re amending, a copy of the 1040-ES payment voucher that went along with your quarterly payment or any IRS payment confirmation showing proof of payment.

Form 1040X cannot be filed electronically; it must be printed and mailed in. A list of IRS mailing addresses is available at the IRS website.
Source: irs.gov