If you have the Auto-updating version, you would have an Icon on your desktop to click.
I'm guessing that you don't, so just head here
http://www.eq2interface.com/download...rnUpdater.html
and download the updater.
Once installed, you will have a "ProfitUI Reborn Updater" Icon on your desktop and Start Menu.
If you run this program every so often, it will download the updates and install them for you.
There is also an option for the updater to start EQ2 when it has finished updating.
If you get stuck anymore, the Author has written a really good FAQ that should help. There is a section for the Auto-updater
http://gm9.eq2interface.com/portal.php?id=15&a=faq
Hope this helps