Overpayment letter from the unemployment office - 5 weeks in

The agency says I reported earnings incorrectly, but I think I followed the instructions. It has been a long 5 weeks. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

Thanks for writing this out. I went through something close to it 5 months ago, and it got easier once I kept a simple log of every call and form.