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The mower belt on my John Deere E130 slips off often. Where do I adjust the tension. It it a 2020 model.

You don't
You need to work out where the belt is jumping from & what it is doing it
If the belt has jumped off a dozen or more times then the belt will be damaged internally.
Check the belt carefully for burns cracks or bits missing
Check all of the pulleys foe some thing stuck in them
Check all of the idlers to make sure they are tightly secured to the deck and turn freely & smoothly.

Thanks. I will do a thorough check of all the above.

If the belt has come off multiple times or has rolled over and run for while. Get a new belt. It can look fine but will continue to come off even if nothing wrong mechanically.

Thanks. Brand new belt but I will check it catefully.

Drop the deck onto a hard clean surface like a concrete drive.
Then get down on the left side ( not discharge side ) and get some one else to start the mower & engage the blades .
Watch where the belt jumps off.
You might need to do this a 1/2 dozen times cause it will happen quickly.
If you can not see an obvious cause get back and tell us exactly where the belt jumps.

It will also be a great help if we know which deck you have and what belt you put on.
JD occasionally change things like the size of the idler pulleys which affects the length of the belt, or the tension on the belt.