Overpayment letter from the unemployment office - first time posting

I received a letter saying I was overpaid about $600 on my claim. I do not usually post, so bear with me. I did not notice any errors at the time. 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.

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.