Overpayment letter from the unemployment office (update)

I received a letter saying I was overpaid about $800 on my claim. There are 4 people counting on me, so I am trying to be careful. 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.

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.