If you need to have your child support payments modified, you typically can by following a child support modification process through the courts or by making an agreement with your ex-spouse. If you can't support the amount of money you're expected to pay, it's important to act quickly to get your modification set into action; if you don't, you could start owing back child support, which can be expensive.
If you know your circumstances are going to change or if they do suddenly due to injury, job loss or other situations, then you need to inform the court. Try to make an agreement with the other parent of your child, asking to modify the amount you owe. If you can agree to a new agreement, even if it's just until you can afford the old payments again, then this will help keep you out of court.
If you're unable to agree, you'll need to keep making the payments to the best of your ability and document why you're unable to pay the rest. These documents need to be kept for the court, so you can show where your income is going besides to your child.
Finally, you can file for child support modification with the court if you can't agree with your ex-spouse. This can take some time, because the paperwork needs to be served to the other parent, and if you can't agree, a court judgment will need to be made. Once this is done, a new child support order can be issued that you'll need to follow in accordance to the court's ruling.
Source: FindLaw, "Child Support Modification Tips," accessed May. 14, 2015