Using American Airlines voucher for a new flight?

This past winter my flight got canceled in the airport and I was issued a voucher for the amount of the ticket. I would like to use that amount toward another flight, but I can't figure out how to do it. On the website, it says to mail it in for a refund, but I want to use it toward a new one and "nonrefundable" is written on the voucher. Please help!