There is no easy way when you think about it.
Gain access to the collar on the shock that adjusts spring tension on the unit and eventually the swing arm.
Rotate clockwise to increase spring tension if this is what you desire.
You may also have a rebound adjustment that can be altered, usually a knob or a screw for adjustment.
I'm pretty sure all you can adjust is preload on that rear shock.(spring)
Verify your adjustment by sitting on the bike prior to replacing side covers which allow access to the shock.
Owner's manual walk off on you?
