For the second time since it released the RTM version of Windows Phone 8.1 and made it available to the public via a Developer Preview program, Microsoft has issued a software update. But the firm has (so far) not explained what's changed.

To be clear, Windows Phone 8.1 is "done." It has already been released to manufacturing (RTM) and if you have used my previously published method for getting the update early, you do have the "final" version of the OS. But Microsoft often adds fixes to its Windows Phone releases after RTM, and of course your handset maker can add firmware and driver updates and apps, and wireless carriers can add apps. So the version of the OS that is deployed to your phone later will be a superset of the RTM version.

This is the second update Microsoft has issued for Windows Phone 8.1. The first arrived about three weeks ago and fixed some battery life issues. It's not clear yet which issues this update addresses.

If you are running Windows Phone 8.1 on your Windows Phone 8-based handset, you get today's update the same way you do with other updates: Navigate to Settings, Phone Updates, and tap Check for Updates.

When it's done, your OS version will be updated to from 8.10.12359.845 to 8.10.12393.890.