Take the strut off and remove the spring. Adjust by pushing the damper rod into the shock body fully and twisting it slowly until it engages with the internal key. Turn to your desired setting.
I have tried doing it on the car by removing the top nut and pushing the rod through the rubber top mounting but the gaiter, metal disc and bump stop all get in the way of adjustment. Even if you succeed with the adjustment, it is difficult to get the damper rod back through the rubber top mount to secure it again.
For me, this is a job to be done the long way!