To switch on the navigation keys, tap Settings > System > Gestures > System navigation > 3-button navigation.
To see all your apps, swipe up from the bottom of the screen.
To go to the home screen, tap the home key. The app you were in stays open in the background.
To see which apps you have open, tap stop. To switch to another open app, swipe right and tap the app. To close all the open apps, swipe right through all the apps, and tap CLEAR ALL.
To go back to the previous screen you were in, tap . Your phone remembers all the apps and websites you've visited since the last time your screen was locked.