Hello @Anna1739,
If you used the pay less upfront option then there's no way on the site for you to pay the remainder early as Airbnb controls the schedule.
If you contact support they may be able to help you pay early.
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.
Steve.