Before starting the update have you first copied off any purchases to your computer's iTunes via File > Transfer Purchases ? You should also backup the device (right-click the iPad 'device' and select 'back up'), and copy off any important documents, files, notes etc (e.g. by email, the file sharing section at the bottom of the Apps tab in iTunes, wifi, whatever the apps that you use support) - the update process should backup and restore the content but it's best to be safe.
What then happens when you connect the iPad to you computer and select it on the left-hand side of iTunes, and on the right-hand side clicked the Check For Updates button the Summary tab ? Does it find an iOS update and start to download it, do you get any error messages ? If it doesn't download the iOS update then you could try it via a browser : https://discussions.apple.com/message/17825148#17825148