Microsoft released Windows 8.1 RTM and Windows RT 8.1 RTM on 17th October for general public to update their Windows 8 system. Many people already updated their Surface RT devices but as an exception, few people reported on Twitter and Forums that they are not able to update their RT devices.
If you are unable to update your Surface RT or unable to find the update in Windows Store, you are one of them who are victim of a small issue reported by Microsoft.
Today in a forum thread, Microsoft support team mentioned that, there is a small number of people who are unable to update their Surface RT devices to Windows RT 8.1 due to a problem. Thus they temporarily removed the Windows RT 8.1 update from the Windows Store and will push it back again once the situation resolves.
If you are one of them, just wait for few days and it will be back again. Here is what Reagan_L (Microsoft Forum Moderator) mentioned in an answer in this forum post: