Told I owe money back on my benefits after 3 years as a call-centre agent

After 13 weeks of benefits I got a notice saying I must repay some of it. I have read a lot of threads here and could not find my exact situation. 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.