How do I amend the 2021 return I just filed?

Do not change or add anything yet.  

You cannot change or add anything on the return that you just e-filed.  It is too late, just like when you put an envelope in a US mailbox.

Now you have to wait until the IRS either rejects or accepts your return.  If your return is Rejected you can go back into your account and fix it, then resubmit it.

But if the IRS Accepts your return you have to wait even longer for the IRS to fully process your return and you get the first Refund.  You will still have to pay the tax due if you owe on it. THEN you can prepare an amended tax return.  The Amended return won't be available until after Feb 24, 2022.