Set the parking brake. Grab the inner sheave on the secondary and rotate it clockwise while pulling up on the belt. The clutch will open and the belt will "fall in". From here, you can hold the clutch open while you work it around to take it off and put the new one on.
Be sure to install the belt so that you can read the numbers from where you're standing on the left side of the sled.
To tension (set the deflection), you will have to loosen the bolts on the secondary that hold the adjuster ring, twist to the appropriate position, then re-tighten. Adjustments should be done with a new belt.
The belt should sit just slightly out of the secondary and have about 1"-1.25" of deflection when you're done.