When or how do I amend my 2021 taxes that has already been sent in?

[xmasbaby0]

MY RETURN IS IN PENDING—what does that mean?

Did you e-file?  Did you click that big orange button that said “Transmit my returns now?”  If so, your return is now in “pending” on a server.  Do not try to change anything while your return is in pending. You cannot change it while it is in pending.   By late January/early February the IRS will begin to accept/reject e-files.   If your return is rejected you will be able to make corrections before re-filing.   If the return you e-filed is accepted you will have to wait until it has been fully processed and then amend it to fix your mistake.

[VolvoGirl]

And you won't be able to amend until much later.  Probably not until late February or later.  You already filed the 2021 return?  Why so early?  We always tell people to wait to file especially since the IRS doesn't start processing returns until the end of Jan.  Last year the IRS didn't open until Feb 12.   So now you have to wait to see if the IRS accepts or rejects your return.  Hopefully it will reject so you can fix it and resend it and not need to amend.   You should not amend until the first return has finished processing and you got the first refund or paid the tax due.

[VolvoGirl]

If your return gets Accepted by the IRS ,   The 2021 Amended 1040X return won’t be ready until Feb 24, 2022. See IRS Forms Availability list
https://ttlc.intuit.com/community/forms/help/irs-forms-availability-table-for-turbotax-individual-pe...

[Critter-3]

AND  if the IRS accepts your return you MUST NOT amend the return until the IRS fully processes the original return  AND  you have the refund in your hand.  

[SweetieJean]

Do you really mean your 2020 return that was filed in 2021?