Your rear shock springs have 5 settings. It's called "Preload" adjustment. It's the only adjustment on your rear shocks.
The shocks have a ring near the bottom that looks like "steps".
Each step that compresses the spring higher/tighter, adds preload, or simply allows the bike to carry a larger amount of weight, but still handle the same, regarding suspension travel and compression of the spring.
Look at your owner's manual for the maximum weight limit for your bike, at full PRELOAD.
There's also a special spanner wrench to adjust the shock and not screw up the chrome. I've already used my hands to adjust them, with the bike on a lift and the shocks extended....
The setting will depend on your weight and that of your passenger. Most sporty shocks will have to be on the highest setting for stock shocks, if carrying a passenger.
The bike comes stock with a solo seat, but a seat for 2-up, and/or a sissy bar are on the accessories list for the bike.