screenmanager.kv

from kivy.app import App
from kivy.lang import Builder
from kivy.uix.screenmanager import ScreenManager, Screen


# Declare both screen
class MenuScreen(Screen):

    def returnstring(self, printstring):
        print printstring

    pass

class SettingsScreen(Screen):
    pass



class ScreenManagerApp(App):

    def build(self):
        sm = ScreenManager()
        sm.add_widget(MenuScreen(name='menu'))
        sm.add_widget(SettingsScreen(name='settings'))

        return sm

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

screenmanager.py

#:kivy 1.5.1
#:import kivy kivy

<MenuScreen>:
    BoxLayout:
        Button:
            text: 'Goto settings'
            on_press: txt_input.focus = False, root.returnstring(txt_input.text)
        Button:
            text: 'Quit'
        TextInput:
            id: txt_input
            multiline: False


<SettingsScreen>:
    BoxLayout:
        Button:
            text: 'My setting button'
        Button:
            text: 'Back to menu'
            on_press: root.manager.current = 'menu'