Microsoft has completed the development process of Windows 8.1 Update 1. Sources familiar with Microsoft’s plans have revealed to The Verge that the software maker recently signed off on a final version of the update, a process known internally as Release to Manufacturing (RTM). Microsoft has started sharing Windows 8.1 Update 1 with partners and PC makers ahead of its release next month. While the update is ready, it’s not clear exactly how Microsoft intends to brand the final version.