I'm sure this is an all-time dumb question, but how do I change the spindle speed from within Mach3?     

I have this XHC-MK4 motion control card:

http://www.automationtechnologiesinc.com/products-page/breakout-boards/cnc-mach3-usb-card-xhc-mk6-4-axis-motion-control-card-usb2-0-interface-wshell

Not sure what you are asking exactly, if it is simply how to call a spindle speed you do that via code or MDI line. S100 would be 100rpm, S1000 would be 1000rpm etc etc.

If you are asking how to actually set up the spindle to be controlled from Mach then you will need to ask the hardware supplier as it will be specific to that hardware.
Hood