Bang on target, Microsoft has gone in and made Windows 8.1 Update available for download. The anticipated feature pack will also arrive via Windows Update later in the day.
But if you cannot wait, then these official links have all the files you need to deploy the new OS. This manual method works for both the 32-bit and 64-bit installations of Windows 8.1. ARM versions like those for the Surface RT and Surface 2 are also available. Here are the links:
Download links for Windows 8.1 Update x86:
Download links for Windows 8.1 Update x64:
Download links for Windows 8.1 Update ARM:
Do keep in mind that KB2919355 is the main package, and if you have an up to date installation of Windows 8.1 that is all you need. If not, also download the other files.
The new version is set to appear on Windows Update around the 10AM Pacific mark.