How do I amend an already filed 2023 federal business tax return?

I assume you mean a corporate return Form 1120 or 1120-S, or a partnership return Form 1065. You will need to correct the return as is needed and then mark the box at the top of the return that says "Amended Return". Then mail it to the address listed in the form instructions according to the state you file in. You can do an internet search for the form number and you will find a link to the IRS website where you will find a link to the instructions for that form.

You should include a copy of the original return with the corrected return when you mail it. Also, write in a description of what you are correcting on the bottom of the first page of the amended return, or attach a memo explaining it.