Overpayment letter from the unemployment office - 5 months and counting

After 6 weeks of benefits I got a notice saying I must repay some of it. 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?

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.