How do I re-define the assembly constraint for the assembly parts in NX? Seems there is no option to define again the constraint. In SW it's always can delete existing mates and re-define the assembly parts.
I does have re-define option,
Right click the assembly component in assembly navigator. Then select "Move"
Move Component pop up and select the Transform - Motion as "By Constraints" from drop down menu.
Hope this resolve your concern.