Update: Microsoft has re-released the Windows 8.1 RT update and you can download it from the Store again.
Some details are available here.
Microsoft has pulled the 8.1 update for RT devices from the Windows Store to address some Windows RT 8.1 upgrade issues that have been occurring. So, if you have a Surface RT, you won’t be able to upgrade until Microsoft sorts that out. They continue to work on it and are making progress.
At this point, Microsoft has released the following statement about RT 8.1:
“Based on our investigations of a situation customers have encountered updating to Windows RT 8.1, we can confirm that as of now this is a Windows update issue only affecting Surface RT customers. While only less than 1 out of every 1,000 (or less than 0.1%) Surface RT customers who have installed Windows RT 8.1 have been impacted, improving their experience and ensuring their systems are fully operable as quickly as possible is our number one priority. We have posted recovery instructions to help affected Surface RT customers recover from the error and complete the Windows RT 8.1 update. We continue to work towards making the Windows RT 8.1 update available in the Windows Store again and apologize for any inconvenience. Further updates will be provided as they become available.”
And here are some instructions on avoiding 8.1 upgrade problems. It won’t help with the RT issue as that’s something broken with the installer but, there are some good practices in there like doing a backup before you start that could help avoid issues if the upgrade goes badly in other ways.
We’ll keep you updated as Microsoft works out the issues; so check back often.