To get the update, just start the existing Ovi Store client on your device and you'll be offered the update. On S60 5th Edition and 3rd Edition devices, you'll have to tap on the downloaded file to install it and accept the 'update' prompts.
The latest update has brought with it a number of improvements, which are detailed in this post on the Ovi blog. These include improved search (priority on 'title' for first search result), the ability to use email address as username when logging in and enablers for stronger DRM and in-app billing. Additionally, the Ovi Store website (store.ovi.com) no longer requires you to select a device when browsing, which means it is easier to see what app and games are available for other handsets.