If my daughters mother claimed her on her taxes while she’s in my physical custody, how can I correct it on my taxes if I didn’t know it and claimed her

If you deserve the deduction, why change your filing, since she is the one who incorrectly claimed the credit?

If you've already confirmed your dependent’s Social Security Number is correct, you'll need to paper-file your return.  The IRS will continue to reject any further attempts to e-file.

Your Federal tax return goes to a different location depending on the state that you live in.  See this list to determine the location for you.   

Where to File Paper Tax Returns (irs.gov)

The IRS will contact both of you to figure out who gets to claim the dependent. (They may take some time to get back to you.)  

The losing party will then be liable for any additional taxes, penalties, and/or interest as a result of the false claim.  

What if somebody already claimed my dependent?