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Previously Microsoft have temporary removed the Windows 8.1 upgrade for Windows RT from the Windows Store. They say there are investigating issues affecting a limited number of users and have removed the upgrade.
I upgraded my Surface RT without any issues and I know other people that have upgraded without any issues.
Hopefully the upgrade will be back soon.

The update to Windows RT 8.1 is once again available as a free download in the Windows Store for users with Windows RT devices. We immediately suspended availability of the update after reports that a very small number of Surface RT customers—around 1 out of every 1,000 Surface RT 8.1 installs—had experienced an issue that left the update incomplete and their devices unbootable. This was due to a rare situation where firmware updates had not completed at the time of the update to RT 8.1. In most cases, if a customer encountered this issue the result was simply an extra reboot. However, for a very small percentage, the boot configuration data was affected which prevented a successful boot. We worked to quickly resolve the issue and now encourage customers to update their Surface RT devices. Surface Pro and 8.1 customers were not impacted by this issue.
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