Some time last week, we announced the features of the upcoming Windows 8.1 GDR1 (Update 1), Microsoft also stated the update will be available this week for those with “developers preview” enabled. It seems they kept their promise and released it today. So if you’re interested in tasting the new features, make sure you have “Preview for developers app” installed, signed in, and enabled for receiving updates. If not, you might not get the “update available” prompt.
If you want to enable developers preview on your phone, here is how you can set it up. Its major benefits is ability to get future OS updates before anyone else,though in the process you lose your device’s warranty.
Now go to Settings—>Phone update—> check for updates. You will be provided with option of downloading via WiFi only or you can opt to download using your Mobile network. The update is said to be around 200 MB, and once its completed you will be informed.
Source: Nokia Power User