How do I report my income for Jan/Feb 2020, if I already filed

You will have to amend the 2020 tax return that was filed to add the additional income.

An amended return, Form 1040X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 16 weeks or longer to process an amended tax return.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

See this TurboTax support FAQ on amending a tax return originally completed and filed using the 2020 TurboTax editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/01/27439

The IRS Form 1040-X for amending a 2020 tax return is scheduled to be available on 03/25/2021 (subject to change)

Please explain.  What income?  Why wasn't it on your return?  Did you miss entering a W2?

You cannot cancel or change or add anything on the return that you 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 will be able to go into your account and add or change it and resend it. Click on Add a State to let you back into your return..

If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return.  Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.

You can check your efiling status here,
https://turbotax.intuit.com/tax-tools/efile-status-lookup/

How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577