Today, June 10, is Microsoft Patch Tuesday. It's also the deadline for consumers running Windows 8.1 to install the Windows 8.1 Update if they want to continue to receive patches and fixes from Microsoft.
Microsoft originally imposed a deadline of May 13 on consumer users to move to the Windows 8.1 Update. On May 12, Microsoft announced a deadline extension to June 10.
Business users still have until August 12 to move to the Update for Windows 8.1 and Windows Server 2012 R2. Microsoft extended the deadline for those updating using Windows Server Update Services (WSUS), Windows Intune, and/or System Center Configuration to update users' machines. Microsoft execs unveiled the new deadline in mid-April after outcry from some that Microsoft wasn't giving them enough lead-time to apply the update.
Consumers running Windows 8.1 who have Automatic Updates turned on don't need to do anything; they should have received the Update automatically via Windows Update by now.
Users who are on Windows 8, not Windows 8.1, are not subject to the requirement to move to Windows 8.1 Update by June 10. (Microsoft has set January 2016 as the end support date for Windows 8 users who haven't moved to 8.1.) Those running Windows 8 who are interested in moving to Windows 8.1 Update do not need to move to Windows 8.1 first, as these updates are cumulative.
The Update for Windows 8.1 included a number of new features designed to make Windows 8.1 more easily navigable by those using keyboards and mice.