By default, apps are automatically installed on your device when a new update is available. Because apps like OnSong are designed for live performance, it's recommended that you disable automatic app updates. This will allow you to install updates when you have time to practice with changes to the apps you rely on.
To do this, go into the Settings app on your device. This is the same app that you open to connect to WiFi networks or to pair Bluetooth devices. Once opened, tap on the iTunes & App Stores option. Locate the Automatic Downloads section and turn off the App Updates option. This will prevent all apps from being automatically installed and will require you to manually install updates instead.
To install updates, open the App Store app from the device's home screen. If you are using an iPad, tap on the Updates tab. If you're using an iPhone or iPod touch, open the Account profile. Pull down on the screen to refresh available updates from the App Store. You can then either update all available apps or choose to update the ones you are ready to test.
Here's a video to help show you how to disable app updates.