Well on my bike the shock body is threaded like a giant screw and the preload is adjustable by changing the vertical position of the spring stop rings which you do by turning them on the shock body.
Use a long screwdriver (with metal all the way through the handle), or other long metal thingie like a punch, and a hammer. Knock loose the outer stop ring by hitting on the extensions. Move it counterclockwise until its loose.
Then hit on and move the inner ring in a clockwise direction while the bike is on a stand. But before all that, measure the distance between the axle (or some point close by) and some point near the seat above it while the bikes on the ground, vertical, without any other weight on it (don't even press down on the seat while holding the bike upright). The measurement come first, then adjust the shock, then remeasure, then keep repeating until the final measurement is less than the first measurement by the amount you wanted to lower the rear end.