Got an overpayment notice, what now?, office manager here

The agency says I reported earnings incorrectly, but I think I followed the instructions. I would rather hear the honest version than the polite one. I did not notice any errors at the time. 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.

If you are unsure, free legal aid in your area may help for this kind of notice.