Got an overpayment notice, what now? after 9 years as an insurance claims processor

After 22 weeks of benefits I got a notice saying I must repay some of it. This is my first time dealing with any of this. 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.