A number of users recently found a secret registry tweak which told Windows to look in a specific location for the Windows 8.1 Update 1 files. This tweak also works on Windows RT, which means you can update your Windows RT hardware to Windows 8.1 Update 1 right now.
As of writing this, the tweak still works. Before we give our instructions, be warned that we do not recommend you try this, and we'd much rather you wait for the official update launch in April.
Firstly, on the Start Screen type "regedit" and hit enter. Navigate to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\" and right click on "Windows", we're going to create a new key. To do this, select New > Key and name it "SHWindowsPoolS14". Once you do that, restart your system and check for Windows Updates. Install the new updates and restart, and then check Windows Update again for a much larger update. This is Update 1. It should weigh in at around 300MB.
Again, do this at your own risk. We're unsure if Microsoft are planning to patch this workaround, and we're also unsure if the update listen in Windows Update is final. So, bare that in mind.