How to have text label update for all Player's Gui??

It's ideal that you use FireAllClients() with a RemoteEvent.

Put a RemoteEvent in ReplicatedStorage, be sure to name it whatever you want and edit the script accordingly.

ServerScript - Be sure to put this in ServerScriptService

local replicatedStorage = game:GetService('ReplicatedStorage')

local Remote = replicatedStorage:WaitForChild('RemoteEvent') -- change to your remote name

Remote:FireAllClients('U are cool')

LocalScript - Be sure to put this in StarterPlayerScripts

local replicatedStorage = game:GetService('ReplicatedStorage')
local Players = game:GetService('Players')
local Player = Players.LocalPlayer

local Remote = replicatedStorage:WaitForChild('RemoteEvent') -- change to your remote name x2

Remote.OnClientEvent:Connect(function(Message)
    local warnUI = Player.PlayerGui:FindFirstChild('Warnui')
    if warnUI then
        warnUI.TextLabel.Text = Message
    end -- check for UI
end) -- on client event