Overpayment letter from the unemployment office

I received a letter saying I was overpaid about $350 on my claim. 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 11 months ago, and it got easier once I kept a simple log of every call and form.

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.