You need a spanner wrench.
^^Looks like that. The tab at the end of the wrench fits into the notches in the collar at the base of you spring. There are tabs on the strut that this collar sits on. You'll see when you look at it, but as you spin the collar with a spanner wrench, it adjust to different "levels" as it sits on the tabs on the strut. As the collar goes up, it gets stiffer, probably what you want.
If you dont have the spanner wrench, it can be done with a punch and hammer, and some patience. The spanner wrench is by far the easiest way to do it though.