Got an overpayment notice, what now? after 13 years as a marketing coordinator

After 6 weeks of benefits I got a notice saying I must repay some of it. I am in Birmingham, in case local advice matters. I did not notice any errors at the time. 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.