R,
I don’t have one here to look at but I think that there is a notched ring around the lower part of the shock that you can engage with a spanner wrench that should be in your tool kit. When you turn this ring it steps the spring pre-load up a notch at a time. You may not be able to see this on yours because of the way the shocks are made, but here’s a picture of what it looks like under the covers.
Scott