Another method is to select the tracks in question, right click on the selected tracks, select "Find Similar Objects", select any further distinguishing criteria in the "Find Similar Objects" dialog box, then change "Any" to "Same" on the "Selected" line, hit "Apply" and "OK", and edit the "Width" field to change the selected tracks to the desired width.
Kinda roundabout, but is a very useful feature when you get used to it.