What tier of Allegro/OrCAD are you running.
Try using Tools, Setup Advisor, Next, Cross-Section.
If you cannot change the impedance and get the trace width calculated then you're running either OrCAD PCB Designer Basics, OrCAD PCB Designer or Allegro PCB Design CIS L. In that case you will have to do it the other way around. Type in a trace width and notice that the impedance is calculated. This way you can interpolate to the impedance you want to and use that in your constraint set.