Unfortunately, I'm not sure whether it is possible to alter the Payment Program conditions (ie monthly payment amount). However, you should be able to make additional payments towards your payment balance:
~taken from above link.
You should note that this is the community forum and not intended as a venue for customer services. If you wish to make additional payments, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.