How do you replace the deck belt on a john deere lt166 freedom 42 deck?

It depends on the Mower Deck you have. I have an LT166 with a Freedom 42" mulcher mower deck. To change the belt, I:

1) Lower the Mower Deck to the bottom position (on the lowest grass height)

2)Remove the front cotter pin and connector that locks the mower deck to the front of the tractor. The connector looks like a square loop with a tail. Slip the mower belt off the tractor (it should be loose and easy to do).

3)Remove the cotter pins that attach the lifter arms (on both sides) to the deck.

4)Remove the cotter pin that attach the blade brake to the tractor. Remove the blade brake shaft from the tractor (it should just slip off)

5) Raise the mower deck lever on the tractor -- the lifter arms should slide out from the mower deck.

6)Slide the mower deck out from under the tractor.

7) Remove the screws that hold the black plastic cover of the deck.

8) Find the spring tensioner on the side of the deck under the cover. Loosen the nut that keeps the spring tensioner tight, until the spring moves freely.

9) Loosen the nut on the FORWARD gear (the one the tensioner is connected to). That gear will then move so you can remove the belt.

10) Replace belt (Make sure the blades on the Freedom deck are aligned first!). Tighten spring tensioner. Tighten nut on forward belt gear.

11) Reverse steps 7,6,5,4,3,2,1