Your wife can petition the court to modify your child support, which is almost always modifiable until your child turns 18 or 19 1/2 so long as he or she is in high school full time. She has the right to go through the courts at any time to seek support. An attorney should be able to calculate child support for you using the same software that the courts use to give you an idea of how much it would be.
The Court has jurisdiction over child support as stated by the prior attorney. Whether the amount of support will change depends on your situation now. Parents cannot waive support for minor children. Therefore, either parent can ask the court to review the support amount provided there has been a change in circumstances. Generally, the courts refer to the Friend of the Court which will want to see your last 2 years of income tax returns and your current pay stubs. Your income information will be considered along with the amount of time that you spend with each child. The state guidelines are mandatory, though the parents can agree to deviate from them. Child support can only be changed prospectively.