You can download the PIA app from the Google Play store here:
Once it downloads and installs, simply launch the application and input your current login credentials. Then, you can choose a server by tapping on "Current Region" right on the main screen above the "Connect!" button. After you choose the server location you want, you can tap that "Connect!" button to connect to that server. If you wish to change servers at any time, you can tap the "Disconnect" button, tap "Current Region", and choose another server from the list. Tap "Connect!" again and you will be connected to that server.