Officially you don't. Games are supposed to have a thingie that lets you run the joystick in a circle to detect the extents. Windows has one too, under the game controller control panel.
Now, if you want to increase sens what you can do is simply not run it to its full extents when it says to move it around in the maximal circle. How the game will handle your input going beyond the expected limit... excitement is the point of living, right?
Less sensitive than the normal extents isn't possible without hardware hacking or some sort of input filter to curve the data.