How to make a Button (Part w/ Click Detector) that gives speed? I am stuck.

Well it is very simple!

local Player = game.Players.LocalPlayer -- getting the player 
local Character = Player.Character -- getting the players' character

local Button = script.Parent -- where the button is located

Button.MouseButton1Click:Connect(function) -- When button is press
    if Player and Character then -- check if player is alive and exist
        -- If alive exist then
        Character.Humanoid.WalkSpeed = 20 -- Change "20" to the speed you want default 16 
    end
end)

Hope it helps!

The question changed soooo:

local Part = script.Parent
local CD = Part.ClickDetector -- CD == ClickDetector

CD.MouseClick:Connect(function(Player))
    local Character = Player.Character
    if Character then
        Character.Humanoid.WalkSpeed = 20 -- Change "20" to the speed you want default 16 
    end
end)

If you want a GUI button, you need to set up some RemoteEvents first, so in ReplicatedStorage you'll put a RemoteEvent called "ChangeSpeedEvent".

Insert a local script inside of the text button and write this in it:

local button = script.Parent
local players = game:GetService("Players")
local replicatedStorage = game:GetService("ReplicatedStorage")

button.MouseButton1Click:Connect(function()

    local player = players.LocalPlayer
    local changeSpeedEvent = replicatedStorage:WaitForChild("ChangeSpeedEvent")

    changeSpeedEvent:FireServer()


end)

and a server script inside ServerScriptService with this in it:

local replicatedStorage = game:GetService("ReplicatedStorage")
local changeSpeedEvent = replicatedStorage:WaitForChild("ChangeSpeedEvent")

local function onChangeSpeedEvent(player)

    local players = game:GetService("Players")

    local char = player.Character or player.CharacterAdded:Wait()

    char:WaitForChild("Humanoid").WalkSpeed = 2

end

changeSpeedEvent.OnServerEvent:Connect(onChangeSpeedEvent)

end)

What that does is listen to when the player clicks a text button from a local script, then it fires the event that you set up previously. The server script listens to when that event has been fired and when it is it gets the character from the player who fired it (who clicked the button) and it sets the WalkSpeed to the number.

If you want a button like a part that you click on, insert a click detector and a server script inside the part and write this inside the script:

local part = script.Parent
local clickdetector = part:waitForChild("ClickDetector")

clickdetector.MouseClick:Connect(function(p)

    local char = p.Character or p.CharacterAdded:wait()

    char:WaitForChild("Humanoid").WalkSpeed = 6

end)

What that does is it gets the player from argument p and gets the character and sets WalkSpeed in Humanoid to a number you want. Hope we could help you.

part.Touched:Connect(function(h)
    local hum = h.Parent:FindFirstChild("Humanoid")
    if hum then
        hum.WalkSpeed = 25 -- change it to whatever you like!
    end
end)

Hello i've used @XviperIink 's code cause then i don't need to remake the whole client sided script:

simple version:

LocalScript >

local Button = script.Parent -- where the button is located

Button.MouseButton1Click:Connect(function) -- When button is press
            local speed = 20 -- u can change this to anything.
        game.ReplicatedStorage:WaitForChild("RemoteEvent"):Fire(speed)
end)

ServerScript >

local remote = Instance.new("RemoteEvent",game.ReplicatedStorage)

remote.OnServerEvent:Connect(function(player,speed)
    local player = workspace:FindFirstChild(player.Name)
    if player then
        player.Humanoid.WalkSpeed = speed
    end
end)