Overpayment letter from the unemployment office, store manager here

I received a letter saying I was overpaid about $450 on my claim. This is my first time dealing with any of this. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

Collect your pay stubs and certification records for the weeks they name. Mine showed the error was a date mix-up.

If you are unsure, free legal aid in your area may help for this kind of notice.