To see which apps you have open, tap the overview key check_box_outline_blank.
To switch to another app, tap the app you want. To close an app, tap the close icon next to it.
To go back to the previous screen you were in, tap the back key . Your phone remembers all the apps and websites you've visited since the last time your screen was locked.
To go to the home screen, tap the home key panorama_fish_eye. The app you were in stays open in the background.