Told I owe money back on my benefits after 8 years as an accountant

I received a letter saying I was overpaid about $1000 on my claim. I am in Fresno, in case local advice matters. 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.