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.