Whenever you access the App Store, it checks for updates to applications you’ve installed. The App Store also automatically checks for updates every week. The App Store icon shows the total number of application updates available.

If an update is available and you access the App Store, the Updates screen appears immediately. Application updates are downloaded and automatically installed when you choose to update them. Application upgrades are new releases that can be purchased or downloaded through the App Store on iPhone or the iTunes Store on your computer.
At the bottom of the screen, tap Updates.
Tap an application to see more information about the update.
Tap Update.
At the bottom of the screen, tap Updates, then tap Update All.
If you try to update an application purchased from a different iTunes Store account, you’re prompted for that account’s ID and password in order to download the update.