As you may know, Microsoft has made Update 1 a mandatory upgrade for existing Windows 8.1 PCs, and a requirement for future updates. And the deadline for that upgrade is today: If you haven't installed Update 1 already, you'll need to install it today through Windows Update.
Microsoft had originally scheduled the all-in date for on May 13, which was the Patch Tuesday date for last month. But based on a slower-than-expected upgrade rate, the firm decided to push the date back to today, which is this month's Patch Tuesday.
Point being, if you want to get updates going forward, you'll need to install Update 1.
A few notes about this update.
Update 1 only applies to Windows 8.1. This requirement is only for those on Windows 8.1. If you're still using Windows 8 for some reason, you will continue to get updates past today. Windows 8 (that is, Windows 8.0) is supported by Microsoft through January 2016.
Today's forced upgrade to Update 1 only applies to individuals. This requirement only applies to individuals running Window s8.1 on their own PCs. Corporations that use Microsoft's deployment tools can push back the Update 1 install through August 12, which of course that month's Patch Tuesday. This will give them additional time to test the update and make sure there are no compatibility issues.
Update 1 is for both Windows 8.1 and Windows RT 8.1. It's not just the x86/x64 versions of Windows 8.1.
You want Update 1. I cannot think of a single good reason to put off this upgrade. Update 1 is an excellent update, full of useful changes to Windows 8.1.
Want to know more? You can learn more about Windows 8.1 Update in my review. Or, man up and purchase my incredibly inexpensive e-book, Windows 8.1 Field Guide, which of course already covers Update 1 functionality. You can find this book on Amazon Kindle for $4.99 or get it directly from me in MOBI, EPUB or PDF formats for just $2 (or you can set your own price).