As a payor of Child support can I decrease my obligation if my ex is now employed with high earnings

You cannot decrease your child support on your own.  If you want to decrease child support, then you must file a petition to modify with the court.  If the child support was 3 years ago or more, then you are entitled to to have child support recalculated under NH law.  If it has not been 3 years, then you most prove the occurence of a substantial change of circumstances which makes the current order unfair.  Even if you ex agrees to reduce child support, you MUST get court approval before it is effective.  If you fail to get court approval, then the current order continues to govern child support no matter what agreement you reach with your ex.