Just call them up, ask for a rate reduction, and if the CSR is unable to do it, request to speak to a supervisor. They'll probably look over your payment history with them, as well as other creditors on the credit reports. Based on that information they'll either grant or decline your request, and probably give you a reason why (if they do decline.) It would also be helpful to prepare some strong arguing points to try to persuade them why they should grant you a lower rate. Ie. something like "my payment history has been perfect"..."I've been a customer for X years".
Good luck.