Microsoft has removed Windows RT 8.1 update from Windows Store due to an undisclosed issue.
Note comes from a user named Reagan_L who is a Microsoft employed Forum Moderator:
Windows RT 8.1. 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.
Thread on Microsoft's forum: