If an update is available for an app on your device, an indicator will appear beside the App Store icon.
Tap the App Store icon, then tap the 'Updates' button in the bar at the bottom of your device.
A list of apps that have available updates will be presented. Tap the Update button beside the app to download the update free of charge.
The update will download, and a small blue indicator will appear beside the app icon when the update is downloaded.