How To increase Walkspeed when you press shift?

I would actually recommend using userinputservice, yes it is more writing but at least you won't bother by checking all the time which numbers were space, control or in your case shift, also i've heard that it doesn't work anymore and im too lazy to check so there is this one:

wait() --- wait for player to load
local human = script.Parent:FindFirstChild("Humanoid") ---- find humanoid inside player
local UserInputService = game:GetService("UserInputService") --- get userinputservice


 UserInputService.InputBegan:Connect(function(inputObject, gameProcessed) -- use userinputservice in a fuction where the button was pressed
     if inputObject.KeyCode == Enum.KeyCode.LeftShift then  --- check if button is shift
        human.WalkSpeed = 60 --- change how fast to move
    end
end)

UserInputService.InputEnded:Connect(function(inputObject, gameProcessed)  -- use userinputservice in a fuction where the button was released
    if inputObject.KeyCode == Enum.KeyCode.LeftShift then -- check if button is shift
        human.WalkSpeed = 16 --- change the speed to normal
    end
end)

NOTE: in order for this script to work it must be in StarterPlayer>StarterCharacterScripts or StarterPack also note out that it must be localscript and it must not stay inside of another model/script/tool/etc.