This article assumes that you already have the QR code of the app that you are trying to view on your phone. If you do not have a QR code and you are in the editor take a look at the instructions here.
Viewing Your App
First open Expo on your phone. You will a see a screen that looks like the screenshot below. Select "Scan QR Code" to begin.
This will open up the camera with a box. Line up the QR code with the box on the screen and Expo will automatically read the code and open your app on the phone.
Viewing another App
If you currently have an app open and want to view another app you will have to go back to the main screen. If you have not navigated anywhere else on your phone you can just click the back button and it should take you back to the expo home page.
Otherwise you will need to open the recent applications menu and select the Expo home page. That will look something like this