If you need to download the Apple iOS manually, do the following. Go to:
Choose the hardware model (e.g. iPad4 (GSM)
Then download the .ipsw file (this can be 2GB in size or more).
Place this file in the following directory (that's for Windows 7):
C:\Users\UserName\AppData\Roaming\Apple Computer\iTunes\iPad Software Updates
When you connect your iOS device to your computer and start iTunes, you can directly load the update.