from kivy.uix.button import Button

class CanvasForm:
    playButton = None

    #def __init__(...):
        self.playButton = Button(text="Play", on_press=self.playButtonClick)

    #def something(...): #def update usually is a good place to check victory/defeat
        #if something: #such as death
            self.playButton.text="Try Again"
            self.add_widget(self.playButton)

    def playButtonClick(self, touch, *largs):
        #do something, then
        self.remove_widget(self.playButton)