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.
Refreshing/Viewing another App
If you currently have an app open and want to refresh the app or view another app you will have to go back to the Expo menu. To view the Expo menu, shake the phone. You will be shown the following menu. Here you can Scan another QR code to view a different app, or refresh the current app to see any updates.