This sounds like it is a rider, if so. loosen the clamp on the main frame then slip the frame to lengthen or shorten the frame. That clamp looks like a muffler clamp and is located between your legs when seated on the mower.

This can depend on the model number of the Snapper rider. Get the model number and check out the units operators manual at the link below. It will go through the adjustments for you.

http://www.snapper.com/us/en/support/man...

There are two set screws in the hub of the pulley mounted to the engine shaft. Loosen and move the pulley closer or farther away from the friction disk. Tighten screws. I have some one push and hold the clutch pedal and leave about 1/16" gap between the two,

There will be a spring belt tentioner built into the mower belt drive engage lever, look up your model for the adjustment instructions.