Overpayment letter from the unemployment office, call-centre agent here

I received a letter saying I was overpaid about $350 on my claim. I worked as a call-centre agent before this. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

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.

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