Got an overpayment notice, what now? after 6 years as a procurement assistant

The agency says I reported earnings incorrectly, but I think I followed the instructions. I have been trying to stay organised about it. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.