There isn't an option to pay the second instalment early as IArbnb dictates the schedule.
You could try contacting them to see if they can process an early payment.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.