Altium: Set width of net class
you can add a rule in the schematic, if you select the "Parameter Set" go to "Add..." and select "Rule" you can add a "Width Constraint" and that will set a rule for all nets in that net class

if you want to actually change the track width itself in the pcb you can use the "PCB Filter" to select all the nets you want with the filter:

(InNet('NetA') or InNet('NetA')) and IsTrack

and then using Ctrl + A to select them all and edit the "Width" in "Properties". Please note that in Altium NEXUS (3.0.13) there is a bug that you have to manually clear this mask in "PCB Filter" or you can't select other things again even if you clear the mask with Ctrl + left click

I'm sure there is a more direct way to select nets and net classes, but this is what i found to be fastest