Microsoft seems to have removed the preview update from Win8 to Win8.1 RT - but this link shows a manual way to update from Win8 to Win8.1 RT.

That update was close to the one I used to go from Win8RT to Win8.1RT on my Surface 2 (I did preview, then the KB updates, then manually installed Win8.1RT).

I installed the OS directly on the device, not via USB. Hope that helps. Weird how you're selling this, but it's not in the sales forum.