I accidentally paid the IRS instead of my state taxes. My payment is set up for tomorrow. What can I do?

Just let it go.   Not much you can do now.

The IRS will see your overpayment  (if there really is one)...and...in a few months, will send you a check to the address on the Federal tax return that your filed.

Then go find your state's website.  Most states have a D.Debit payment page where you can set up a payment directly with them.   They also have payment voucher forms that you can fill out and mail in with a check if you want to do it that way instead.

Still, I'd wait a few days to see if the IRS payment happens, and make sure it looks like the IRS or US Treasury pulled the payment when you check your account transactions.  Plenty of time remains to set up the proper State payement