Please refer to this link if you are Downloading an app for the first time .
To Un-install an application from your device and your iTunes library:
- Touch and hold any application icon on the Home Screen until the icons start to wiggle.
- Tap the "x" in the corner of the application you want to delete.
- Tap Delete to remove the application and all of its data from your device.
- Optional: Remove an App Store application from your iTunes library:
- Click on Applications from the left-hand column of the iTunes window.
- Locate the application and either right-click or Control-click and choose Delete from the shortcut menu.
- Select Remove when asked if you want to remove the selected application from your iTunes library.
- Select Move to Trash when asked if you want to move the selected application to the Trash or Recycle Bin.
To Download the application to your device or computer again:
- Go to the App Store on your device or in iTunes.
- Make sure you are signed in with the same account used for the original purchase.
- Locate the application and click the "Buy App" button or tap the price on the device. (For newer OS versions, previously purchased apps simply display an 'Install' button instead of the price. )
- You will then see a dialog similar to one of the two below.
Redownloading from your computer:
Redownloading from your device: