When a player dies or leaves the server how do you make a textbox change? [closed]

For death:

Every character has a humanoid, and humanoids has a .Died event. To check every character that has joined the game, you also need to check every player that gets in the game since your script is server script.

Here's PlayerAdded event of game.Players

Here's CharacterAdded event of Player

And Here's Died event of Humanoid

And here's an example

game.Players.PlayerAdded:connect(function(Player)
    Lives = 3
    Player.CharacterAdded:connect(function(Char)
        Char.Humanoid.Died:connect(function()
            Lives = Lives - 1
            if Lives == 0 then
                print("Game Over")
            end
        end
    end
end

For Leaving:

For this, we need to use game.Players' PlayerRemoving event.

And here's an example

game.Players.PlayerRemoving:connect(function(Player)
    print(Player.Name.." is leaving the server.")
end

Additional note: Keep in mind that putting wait(Time) after a PlayerRemoving event will cause an error if you try to work on that player since Server can't keep removed player's information that much.