Sign into an existing account

It looks like he grabbed this tablet from a restaurant, and it’s screen is like what you find at an airport, it’s dedicated solely to the use of one app, which is in this case a food delivery app. How would I go about returning it to “personal use” with YouTube access and all that. At the moment, you can only close the food delivery screen for about a half of a second before the app snaps back on screen. Help?