Using Windows 8.1? It's time to get the update

On Tuesday, Microsoft will make Windows 8.1 Update available. To continue to get patches, 8.1 users need to move to it soon.

As of Tuesday, all Windows 8.1 users will be able to move to Windows 8.1 Update.
Microsoft will be making the update -- formerly known by those of us who've been tracking it for a while now as "Update 1" and/or "Spring Update" -- available via Windows Update.
The update, designed primarily to make Windows 8 more palatable for mouse/keyboard users, will be designated as an "Important" security update when it's rolled out. It's actually very important, as Microsoft is going to require all Windows 8.1 users to have Windows 8.1 Update installed if they plan to continue to download any security or feature updates to the operating system.
Microsoft's Premier Field Engineering blog, in an April 7 post outlining the update, makes this plain:
"Failure to install this (Windows 8.1) Update will prevent Windows Update from patching your system with any future updates starting with Updates released in May 2014 (get busy!)"
A Microsoft Springboard Series blog post from April 2 also emphasizes that Windows 8.1 users need to move to Update to continue getting patches.