Generally, this means that at least one of these things has happened:
The income of the parent ordered to pay child support has either increased or decreased, or
The parent ordered to pay child support is legally responsible for additional children, or
The child's medical insurance coverage has changed, or
The child's living arrangements have changed.
Warning: Do not file a frivolous modification suit. You can be assessed attorney’s fees if the court finds that the modification suit was filed frivolously or to harass the other side. See Texas Family Code
Some Texas courts have said that a decrease in a parent's salary can be a material and substantial change in circumstances supporting a modification in child support. To help you figure out if you could succeed in your modification suit, you should talk to a lawyer who practices in the county where your orders were signed.