You need a solid internet connection and uninterrupted time for the upgrade. After it successfully downloads to the computer, it should ask you, again, if you want to upgrade the iPad.
Before you do the actual update you want to be sure you did a backup.
Once you start the update you do not want any interruptions, so make sure your computer is plugged into power (if you were using a laptop) and your iPad has plenty of charge, 50% at the minimum, I'd say.
If for some reason the iPad does not complete it's upgrade you'll have to restore it and try again. That is why you really want that backup.