I got the message several weeks ago, and tried to do the update on my old Kindle ebook. It wouldn’t update anything, so I’ll wait and see what happens. Couldn’t find a relevant update for my Fire. And the message didn’t specify which machine they were talking about.
Poorly managed process from Amazon.
From "home" go to "menu" then "sync and check items". If it doesn't update it probably already did.
What model Kindle do you have? It may have already done the software update automatically if you have the WiFi turned on. Current Kindle software version history here: http://www.amazon.com/gp/help/customer/display.html/ref=hp_bc_nav?ie=UTF8&nodeId=200529680
General information:
https://en.wikipedia.org/wiki/Amazon_Kindle