You can goto Billing->Manage Account and than edit your existing plan. You can do some modification if necessary and than restart a plan as new. The remaining credit will transfer to your new plan and any outstanding amount will be billed to you.
If you do not have to change anything, you can just goto Billing->Make Payment and enter the amount manually and PAID them as pedrotuga suggested.
FULL_97_OFF <-CLICK = host 1 yr for $22.4