You have to submit a motion for modification of child support. The motion is styled with the heading of the case, as demonstrated on the Order for child support (case no., court, parties names, etc.). In your motion, (called a Memorandum-In-Support) you'll want to lay out the events of the case leading up to the current order. Then explain the events or circumstances why you believe a modification is appropriate, and then make your motion for a modification. You'll need a separate draft order which essentially says what you want the Judge to order, which will look similar to the current order.
A copy of the motion will go to your ex- or his attorney, if you know he has an attorney. There are certain minimum time periods (usually 10 days) you'll need to serve your ex- with a copy, and then you'll file the original with the Family Court Clerk's office. Sending the copy to your ex- certified with return receipt, will be a good idea. Along with the motion, you will also want to do a Notice, indicating you would like a hearing, if you desire one, which you'll want to Notice at least 2 - 3 weeks out. Talk with the clerk's office for specifics on motion hours and filing deadlines for the next motion hour. Otherwise, you can file your motion with the family court clerk's office, still observing the applicable minimal time periods, and the judge will make a ruling. The clerk's office may also have forms which may simplify your efforts to file a motion for modification.
You would be better served hiring an attorney, but these are the basics for preparing and filing a motion for modification. There may be other procedural steps which may be required that an attorney would better understand and have knowledge of. Good luck!