Got an overpayment notice, what now?, IT help-desk technician here

I received a letter saying I was overpaid about $350 on my claim. I have read a lot of threads here and could not find my exact situation. 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.