You can always find the latest APK for your version of Xibo for Android in the Spring Signage Download Centre.
Download the APK directly to your device, or alternatively transfer it to the device using the same method you used for installation.
Once the file is on the device, press on it to launch the upgrade. A box will appear informing you that the app you're about to install will replace an existing application:
Press "OK" to allow the upgrade to complete.
Once the upgrade has finished, launch Xibo for Android in the normal way. You can check you have the latest version by looking at the status screen.