The Amazon Kindle is a pretty amazing device, bringing back the joy of reading in a handy and portable form which can also store thousands of books at a time. If you're still using an older model, any of those released before 2013, then you'll really want to grab the latest update.
If you don't update those older models you'll be unable to head online. That will cut off users from adding new titles from the store, and even accessing books you've previously bought which are stored in the cloud. So it's a pretty important thing not to miss.
The update has to be applied by the 22nd of March in order to retain the online functionality and should be automatically applied if you are logged in with Wi Fi or data. All models from the original version in 2007 to the 2012 5th Generation Paperwhite. If you do happen to miss the update, you will be able to fix your Kindle by plugging it in via USB.