I just learned about this today. Apparently today is the last day so I will be doing this tonight.
"To update a Kindle, owners should connect the devices to Wi-Fi and then tap "menu" or the menu icon, then select "sync and check for items." Once the software update is competed, owners should get a letter titled "03-2016 successful update" on their devices, which Amazon says can be found by sorting for the most recent items in their library."