Yes you can and you would go straight to 5.1 now. You need to download the latest iTunes on your computer and backup everything. It's a good idea to disable your virus protection and/or firewall for the duration of the update. It is a very big update so can take some time depending on what you already have on your ipad. Have a look at this link and it will show you how to do it.
[video=vimeo;30421920]http://vimeo.com/30421920[/video]
Did you forget to add the link?
I'm downloading the update to my laptop now, they say it will take more than an hour and not to touch it.