How do I change the GUI logo and textbox for everyone?

You will have to synchronize all the scoreboards using a global scoreboard if you want an easy version. That works like this:

-- Setting the example leaderboard.
global = Instance.new("Folder", workspace)
global.Name = "GlobalScoreboard"
points = Instance.new("IntValue", global)
points.Name = "Points"
logo = Instance.new("ImageLabel", global)

function Update()
	local plrs = game.Players:GetPlayers() -- Returns a table with all the players.
	for _, plr in pairs(plrs) do -- Will do the same for all players.
		-- Destroying the old ScoreBoard
		plr.ScoreBoard:Destroy()
		-- Setting the new ScoreBoard
		local new = global:Clone()
		new.Parent = plr
		new.Name = "ScoreBoard"
	end
end

points.Changed:connect(Update) -- Event
logo.Changed:connect(Update) -- Event
global.Changed:connect(Update) -- Event

Hope this helps you with the understanding and good luck with the game!