Officially, Windows Phone 8.1 won't be available on existing Windows Phone 8 handsets for a few months, but there's nothing stopping users from installing the update right now.
To install the update, you need developer status, but that's easy enough to achieve. Just create a new project using Windows Phone App Studio, a free app creation tool on Microsoft's Website. You don't have to design or ship a complete app; as long as you've started a new project, that's good enough. You'll also need Windows Phone 8 Update 2 installed on your phone.
After you've created a project, download the free " Preview for Developers" app for Windows Phone, and opt into receiving updates. You may want to opt out of pre-release versions so your phone doesn't end up with buggy code. According to SuperSite for Windows, you should be able to find the update by going to Settings > Phone Update > Check for Updates.
Keep in mind that once you've updated to Windows Phone 8.1 through the developer preview, there's no going back. Also, as The Verge points out, your warranty will be invalid until the official update arrives through your wireless carrier. Any features added by device makers, such as Nokia, won't be part of the update either.
Still, there are plenty of good reasons to update now, from the Cortana voice assistant to Action Center for notifications to the Swype-style Word Flow keyboard. And if your wireless carrier has abandoned support for your phone (as T-Mobile has done with the Lumia 810), this is the only way to stay current with Windows Phone.
One more thing: Users outside of the United States won't have access to Cortana by default. It's possible, however, to get Cortana by going to Settings > Region and selecting United States in the "Country/Region" box.