Before you update your iPad to iOS 5, make sure you have all your information backed up to iTunes or iCloud. You can then choose to update your iPad through Wi-Fi or by plugging it into your computer and accessing the update through iTunes. To update via Wi-Fi, make sure you have a good battery charge and go to Settings>General>Software Update. The iOS will see what updates are available and will start the update process by downloading the update. When the download has completely finished, tap Install.

If your VPN or proxy connections do not allow you to update through Wi-Fi or if you prefer to update through iTunes, plug your iPad into your computer, open iTunes, choose your iPad from the left hand navigation under Devices and then choose “Check for Updates”. Keep your iPad plugged in until the update has completely finished.

Resource: Apple Support