There are certain aspects of a divorce decree that can be modified later and certain aspects that cannot. Issues regarding the children (conservatorship, possession and access, child support, health insurance, and almost any other issues related to the children) can be addressed in a modification. To modify, one party needs to file a Petition to Modify Parent-Child Relationship. There is no waiting period for a modification, so if the parties are in agreement, a modification order can be entered almost immediately after the petition is filed. If the parties are not in agreement, the case would follow the standard child custody case procedures.
You cannot, however, modify property or debt division from a divorce decree. For this reason, property and debt division should never be tied to child support or custody issues. (For example, if one party agrees to take a smaller piece of the pie in the property division in exchange for receiving higher monthly child support, this would be a huge mistake. The monthly child support amount can always be modified later, but the person who took the smaller piece of the property pie can never go back and reclaim what was rightfully his or hers.) This is also a very key reason why parties should consult with attorneys before agreeing on property or debt issues in a divorce. I have seen it time and time again where someone makes a critical mistake with regards to property (usually a house) in a pro se divorce decree. They have no way of fixing the problem down the road because you cannot modify a decree on that issue.