How do I change a surface GUI with a click detector?

How do I change a surface GUI with a click detector? can anybody help me? I am trying to make a boss fight for a simulator: here is what I am thinking: I put a click detector in my part that is the boss when you click it there is a sign that changes its text (when you click it, it changes to "Health - <number of health>") and when you clicked the boss enough the sign changes back to its regular health and rewards you with 4000 leaderstats. here is my script so far (i have only made the when clicked change the sign to say something)

game.Workspace.Gummy.ClickDetector.MouseClick:connect(function()
    Text = ("health")
end)



--variables
Text = script.Parent.Parent.Sign.Text.SurfaceGui.TextLabel.Text

can anybody help me?