Microsoft has released Windows 8.1 Update 1 to manufacturing, according to my sources, as well as the known Windows leaker WZor.

Windows 8.1 Update 1 -- which Microsoft also has referred to internally as "Windows 8.1 Spring Update" and "Windows 8.1 Update" -- is designed to make Windows 8.1 more palatable and familiar to users who prefer to use mice as much, if not more, than touch.

Word of Windows 8.1 Update 1's existence and feature set began leaking months ago. Pre-RTM builds of the update have been leaking for weeks. (The RTM build has not yet leaked to the Web as of March 4.) During Mobile World Congress, Microsoft execs officially provided some information on the coming release for the first time publicly .

Windows 8.1 Update 1 tweaks include a title bar at the top of Modern/Windows Store apps; new, right-clickable context menus; and the addition of power and search buttons on the Start Screen. Windows 8.1 Update 1 will have a smaller footprint (1 GB of RAM; 16 GB of storage), which will enable device makers to put Windows 8.1 with the update on smaller, cheaper tablets. Windows 8.1 Update 1 will include IE8 compatibility mode in IE11 -- previously reported as IE11 Enterprise Mode .

CNET: Windows 8.1 update aims to fix common issues

With Windows 8.1, according to previous reports, boot to desktop will be installed by default on new PCs/devices without touch screens. Users who are upgrading from Windows 8.1 to Windows 8.1 Update 1 who don't have boot to desktop set as their default already won't see their settings change to boot to default, according to WZor.

WZor reported that the build designated RTM was compiled on February 21 and signed off on February 26. Microsoft's next step is to provide the RTM code to OEMs for preloading on new machines.

Microsoft also has released the complementary Windows Server 2012 R2 Update 1 to manufacturing, WZor said. Microsoft officials acknowledged last week that Update 1 would be coming to server as well as client.

Microsoft is planning to make Windows 8.1 Update 1 available to MSDN subscribers on April 2, according to Windows SuperSite's Paul Thurrott. Microsoft will make the update generally available via Windows Update on April 8, Patch Tuesday , I've heard from my contacts.

Microsoft may release at least one more update to Windows 8.1 later this year. I've heard from my contacts plans as to whether or not to do this are still not set in stone inside the company.