Overpayment letter from the unemployment office - 4 months and counting

After 15 weeks of benefits I got a notice saying I must repay some of it. I worked as a delivery driver before this. I did not notice any errors at the time. Who should I talk to first?

Thanks for writing this out. I went through something close to it 4 months ago, and it got easier once I kept a simple log of every call and form.