Told I owe money back on my benefits after 16 years as a software tester

The agency says I reported earnings incorrectly, but I think I followed the instructions. I worked as a software tester before this. 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.