At 7 am ET on Thursday, October 17, 2013, Microsoft began delivering Windows 8.1 to customers. The update is available now for free to existing users of Windows 8 and Windows RT. And the retail version will appear in stores, and online, starting tomorrow, on Friday, October 18, 2013.

"Today we are announcing the global availability of Windows 8.1," a Microsoft statement notes. "Windows 8.1 demonstrates our commitment to continuously improving [Windows] to create a richer customer experience. We are excited to have customers start updating their devices today and getting to experience new Windows devices this holiday season."

While the name suggests a minor update, Windows 8.1 is in fact a not-so-subtle step back from the hole Microsoft dug for itself in Windows 8 a year ago. That release was incomplete, and provided users of both traditional PCs and newfangled tablets and hybrid PCs with a less-than-ideal experience. In Windows 8.1, a slew of minor changes—and a few profound ones, too-dramatically improve things. And that's true for all users, regardless of the type of PC they use.

You can read all about that in my massive Windows 8.1 Review on the SuperSite for Windows. But here's what's happening today (and tomorrow).

Electronic availability. If you're running Windows 8 or the Windows 8.1 Preview, you can trigger the update electronically by visiting this Microsoft link. Users with Windows XP, Vista, or 7 should visit the Buy page on Windows.com for find out about their options. Windows RT users can trigger the download by visiting the Windows Store.

Retail availability. If you would like to buy the retail version of Windows 8 ("Core") or Windows 8 Pro, you can do so starting on Friday. Or, you can preorder either product today from the Microsoft Store and other retailers.

New PCs and devices. As you might expect, Microsoft and its PC maker partners are releasing a new generation of Windows 8.1 PCs and devices starting immediately. Visit the Windows web site to learn more.

GA Rollup A. As described previously on the SuperSite, a major rollup of post-RTM fixes to Windows 8.1 called GA Rollup A will ship to customers via Windows Update within the next few days.

New apps. In addition to updating all of the apps that ship with Windows 8.x, a number of important high profile apps are also shipping today for Windows 8.1 in new or updated form, including Facebook, Hulu Plus, Evernote, eBay, Netflix, NOOK, Adobe Photoshop Express, Adobe Reader Touch, and more.

And of course, stay tuned to the SuperSite for Windows, where you will find ongoing coverage of the new Windows, including my Windows 8.1 Tips and Hands-On with Windows 8.1 series, and regular updates to my new e-book in progress, "Windows 8.1 Book."