Windows 8.1 is available for download for free for Windows 8 users. Here’s how to get it.
If you’re running on older versions of Windows, you’ll need to pay for the upgrade to Windows 8.1, but existing Windows 8 users can upgrade for only the cost of the data. Mind you, that’s no small order. You’re looking at multiple gigabytes to upgrade, so if you’re on a slower connection it could take some time.
If you’re up to date with Microsoft’s Windows Updates, you should find the Windows 8.1 update sitting nicely in the Windows Store when you open that up, ready to install. Having said that, as with any operating system upgrade, it would be wise to back up any vital files — your own documents, primarily — before running the upgrade, as well as giving yourself some downtime for the upgrade process to actually run.
What if it’s not there? The most likely culprit is that you’re not up to date with needed Windows Update files. Run Update and get yourself prepared, and then the Windows 8.1 update should arrive in the Windows Store once you’ve restarted.
How Do I? covers the basics, because we’ve all got to start somewhere.