Updating a player's gui help?

So I know that you can't change the startergui cause that's just inserted into the character when they join, how would I access the players gui and update this script from there

	for timewait = 2, 0, -1 do
		Players = game.Players:GetPlayers() --Define players
		wait(1)
		game.Workspace.StartingLine.Timer.SurfaceGui.Time.Text = "Picking Terrain"

		game.StarterGui.MainGui.TimeFrame.Timer.Text = "Picking Terrain" --The part that needs to go to the playergui not startergui

	endi
	end

To answer your question quickly, you never cycle through the players, only change the starting gui, not their gui. To fix use this script:

for timeWait = 2, 0, -1 do
	for _, Player in pairs(game.Players:GetPlayers()) do
		Player.PlayerGui.MainGui.TimeFrame.Timer.Text = "Picking Terrain"
	end
	workspace.StartingLine.Time.SurfaceGui.Time.Text = "Picking Terrain"
	wait(1)
end