如何在kivy中更改标签文本？

from kivy.app import App
from kivy.uix.label import Label

class MyApp(App):
    def build(self):
        pass

class MyApp(App):
    def build(self):
        label = Label(text='初始文本')
        return label

class MyApp(App):
    def build(self):
        label = Label(text='初始文本')
        label.text = '新文本'
        return label

from kivy.app import App
from kivy.uix.label import Label

class MyApp(App):
    def build(self):
        label = Label(text='初始文本')
        label.text = '新文本'
        return label

if __name__ == '__main__':
    MyApp().run()