Hi,
You can try the following things:
1) Single tap the app’s icon on the home screen and it will pause the download. Now go back to the iTunes Apps store, switch to the “Updates” screen and click “Update” to resume the download.
2) If this doesn't work, go to Settings -> iTunes & App Stores and tap the Apple ID to sign-out. Restart the iPad, go back to Settings -> iTunes Apps Store and sign-in. Tap the waiting icon to update the app.
Do let us know if this helped.
-Charu