Just as promised, Windows Phone 8.1 is now available for download to developers. Joe Belfiore, the chief of the Windows Phone Program Management at Microsoft has tweeted a screenshot of the WP 8.1 update prompt.
Mind you, before being able to install the Windows Phone 8.1 update, you'll have to download a minor update that prepares your phone for the essential one. Regular consumers can expect their phone to receive the update in a few months, while the first devices to run it out of box will come at the end of this or the start of next month.
If you are in a hurry, registering as a Windows Phone developer isn't really that hard. You can either pay $19 to Microsoft and get access to publishing apps in the Windows Phone Store or register as a developer through App Studio. The latter option is free and is aimed at enthusiasts but it also grant you the Developer status, which in turn lets you install the update.
The Windows Phone 8.1 update brings a lot of new features to Microsoft's phone OS. Most notably, those are the digital personal assistant Cortana, Action center, updated UI with custom lockscreen and tile backgrounds, IE 11 as well as updated security and Sense apps.
8.1 windows download
Windows 8.1 software download
My phone download software