My I pad has a iOS 4.3.5, how can I update to iOS 5...?

Update it from iTunes on a computer as described at the bottom of this article. As it's running an iOS version prior to 5.0, there won't be an option to update from the device itself.

The third generation iPod touch and original iPad can’t be updated past 5.1.1.

(80827)

Connect your iPad to your computer then launch iTunes then follow the prompts to update.