--Variables
local player = game.Players.LocalPlayer
local uis = game:GetService("UserInputService")

--Functions
uis.InputBegan:Connect(function(input)

    if input.KeyCode == Enum.KeyCode.LeftShift or input.KeyCode == Enum.KeyCode.RightShift then --Change "LeftShift" and/or "RightShift" to your desired keycode
        player.Character.Humanoid.WalkSpeed = 50 --Change "50" to your desired walkspeed (Default WalkSpeed = 16)
    end

    uis.InputEnded:Connect(function(input)
        if input.KeyCode == Enum.KeyCode.LeftShift or input.KeyCode == Enum.KeyCode.RightShift then
            player.Charactercharacter.Humanoid.WalkSpeed = 16
        end
    end)

end)

-- Help regarding Enum.KeyCode: https://create.roblox.com/docs/reference/engine/enums/KeyCode