The best way to get the update is to check your Kindle Fires About menu (Settings ->Device -> About) and click the button there. But if you want to download the update yourself, here are the links:
The 3 tablets each have their own update and they're not quite getting all the same features. For example, the two 7" models are gaining support for Simplified Chinese, a feature which is already available on the KFHD 8.9.
But generally the 3 tablets are getting the same upgrades, and as I look over the list I'm pretty sure Amazon is planning to make a big push into schools. Three of the 4 new features are going to make these tablets more attractive to teachers, IMO.
X-Ray for Textbooks, for example, adds value by giving students more useful info at the tap of a finger. They can access all the most important terms and concepts in a textbook, with glossary definitions and links to relevant textbook pages. And with improved support for Kindle Print Replica (Amazon's PDF format), students will be better able to navigate and read textbooks on their Kindle Fire.
But the one really cool update today is something I never expected to see. Way back in 2010 Amazon gave their Kindle for iOS apps the ability to play audio and video clips. Thanks to this recent update, the Kindle Fire HD can now support this enhanced content. (Or at least it is supposed to be able to play the videos; I cannot get them to work on my KFHD.)
Video and audio enhancements in ebooks don't appear to be amounting to much in the consumer market, but the same cannot be said for the academic market. There is also a lot of hype surrounding iBooks and how it supports video, audio, and other enhancements, so I can understand why Amazon would feel that they have to match Apple.
This also ties in closely with the recent news that Amazon was enabling bulk purchases of Android apps. That is an ability schools will need in order to manage a large quantity of Kindle Fires.