As noted in the Owner's Manual it is recommended that the brakes on this model be adjusted by a professional to be sure they work to their full potential without dragging and wearing excessively.
If you do want to try adjusting them you will see two rods going back to the transaxle on the right hand side (just behind the discharge for the mower). Those go to the brake assembly, which is a small disc on the side of the transaxle.
The brake can be adjusted tighter or looser by turning the nuts against the spring. As the rod is tightened or loosened it will alter the point where the pad moves against the disc when you step on the pedal.
These adjustments can also alter the clutch engagement since the same pedal operates both. The measurements can be critical for everything to operate properly, reliably, and safely which is why it is recommended a professional perform any adjustments needed.