It is a common occurrence for apps to occasionally get stuck on “waiting” when they are updating or being downloaded for the first time. If this has happened to you, there are several things you can to do stop the app and get it to work properly. Downloading apps in iTunes first and then syncing them to your iPhone is usually one sure way to make sure that apps don’t get stuck on waiting when you are downloading them directly to your iPhone. However, for a quick fix, try restarting your iPhone first to see if that will reset the app to make it work the right way. Also, make sure your iPhone is charged and has a good connection.
If you do get an app that is stuck, tap on the icon that appears on your Home screen for the waiting app until the icon starts shaking in order to access the “Delete” option so you can delete the app. Then, reinstall it from iTunes. You may lose some options for that app, but you will be able to reset those once the app is back on your iPhone.
If your iPhone app is stuck on waiting while it is trying to update, tap on it once to pause the download. Once the app is paused, go to the iTunes Apps store and go the “Updates” screen and choose update. At that point the app will resume downloading.
Source iTurfApps