To access jump power you can use both a local and server script.
For a server script:
game.Players.PlayerAdded:Connect(function(player)
player.CharacterAdded:Connect(function(character)
local humanoid = character.Humanoid
humanoid.JumpPower = 0 --power u want
end)
end)
In a local script:
local player = game.Players.LocalPlayer
local character = player.Character
local humanoid = character.Humanoid
humanoid.JumpPower = 0 --desired jump power