My LLC has changed from multimember to single member LLC effective 01/01/23 in the state of Alabama. Partner retired. How to register this change?

You will need to obtain a new employer identification number (EIN) for your single-member LLC and close out the one you have now. To close out your current EIN, you will need to send a letter to the IRS listing the company name, address, EIN and reason you are no longer using the old EIN. You will send the letter to:

Internal Revenue Service

Cincinnati, OH 45999

You can read more here:  Close business

You need to report your 2023 income and expenses on your personal federal tax return as self-employment income, as if you were filing as a sole proprietor. You must first apply for a new federal ID number for your single-member LLC and enter it in TurboTax when you report your business income and expenses.  Here is a link on applying for a new EIN:  Apply for EIN

When you filed your last Form 1065, you should have checked the Final Return box, but if you failed to do so, it may not be a problem unless you are contacted by the IRS about it. If that happens, you can explain what happened and that should resolve the issue. 

You should contact the business licensing department in Alabama to see if you need to do any paperwork to properly report the change of ownership of the partnership, but that is beyond the scope of the tax advice we give here.

[Edited 3/5/24 at 10:38 AM PST] @kdelo