how to change walkspeed in this script when crouching???

So i made a crouch script that makes you crouch. i got a idle animation as well when you aren't walking. i know how to change walkspeed but not where to put it in this script. Can somebody help me?

Here is the script:

ps i have a RemoteEvent that is named "OnCrouchBegun"

local uis = game:GetService("UserInputService")

local crawlAnimation = script:WaitForChild("Crawl")
local loadedCrawlAnim

local crawlIdle = script:WaitForChild("CrawlIdle")
local loadedIdleAnim

local isCrawling = false

local humanoid = script.Parent:FindFirstChild("Humanoid")


uis.InputBegan:Connect(function(key, gameProcessed)

    if key.KeyCode == Enum.KeyCode.C then

        game.StarterPlayer.CharacterWalkSpeed = 5

        if isCrawling then

            isCrawling = false  

            if loadedCrawlAnim then loadedCrawlAnim:Stop() end  

            loadedIdleAnim:Stop()           

            game.ReplicatedStorage.OnCrouchBegun:FireServer(humanoid, 16, 50)

        elseif not isCrawling then



            isCrawling = true   

            loadedIdleAnim = humanoid:LoadAnimation(crawlIdle)
            loadedIdleAnim:Play()

            game.ReplicatedStorage.OnCrouchBegun:FireServer(humanoid, 3, 20)
        end
    end 
end)


game:GetService("RunService").RenderStepped:Connect(function()

    if not isCrawling then return end

    if humanoid.MoveDirection.Magnitude > 0 then

        if not loadedCrawlAnim then loadedCrawlAnim = humanoid:LoadAnimation(crawlAnimation) end
        if loadedCrawlAnim.IsPlaying == false then loadedCrawlAnim:Play() end   

    else

        loadedCrawlAnim:Stop()
        loadedIdleAnim:Play()
    end
end)