Microsoft has apparently removed the Windows RT 8.1 update from the Windows Store, citing a situation as the culprit for some users. Microsoft is investigating the issue and will provide further details once the update is ready to be released again.
"Microsoft is investigating a situation affecting a limited number of users updating their Windows RT devices to Windows RT 8.1. As a result, we have temporarily removed the Windows RT 8.1 update from the Windows Store. We are working to resolve the situation as quickly as possible and apologize for any inconvenience. We will provide updates as they become available," Microsoft stated in an official support forum.
It's currently unclear when Microsoft is going to push the update out again. It seems the majority of users who updated via the Windows Store had no issues but a select few are experiencing some troubles. Microsoft rolled out the update on October 17th.
According to an unofficial source, the update fails due to a boot configuration error. After the update installs and reboots your Surface RT, you are greeted with a nice blue screen that says, "Your PC needs to be repaired. The Boot Configuration Data file is missing some required information." Heres a guide on how to remedy the situation, which requires the use of a bootable USB recovery drive.
Thanks for the tip, Super User!