To sign in/out of additional user accounts set up on the device, view this info.
With the device powered on, press the Power button twice to access the lock screen.
Tap the desired user icon (located at the top). The active user is designated by a circle around the user icon. If prompted, enter the user account password/PIN/pattern or swipe the screen or scan fingerprint to unlock.
To sign out, simply press the Power button to power off the device.