how to make a script that change a value when you jump ?

You can use either Jumping or StateChanged. They are both events of a Humanoid that can be used to tell when a Humanoid is jumping. Because they are events, you would connect them to functions, like so:

Using Jumping:

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function(character)
		character.Humanoid.Jumping:Connect(function()
			-- Code here
		end)
	end)
end)

Using StateChanged, which fires with two arguments, the original state and the new state of the Humanoid:

game.Players.PlayerAdded:Connect(function(player)
	player.CharacterAdded:Connect(function(character)
		character.Humanoid.StateChanged:Connect(function(originalState, newState)
			-- Code here
		end)
	end)
end)

By using the code given, you can expand upon what I gave you as examples to accomplish what you're looking to do.