Well, while Microsoft announced free upgrade to Windows 10 for all Windows 8.1, Windows Phone 8.1 and Windows 7 devices, there was no word about the current Windows RT and Windows RT 8.1 tablets like Lumia 2520. So, now it seems Windows RT and Windows RT 8.1 devices are not upgradable to windows 10 as far as the available official info is concerned. The Windows 10 page of Microsoft tells us that,
*It is our intent that most of these devices will qualify, but some hardware/software requirements apply and feature availability may vary by device. Devices must be connected to the internet and have Windows Update enabled. ISP fees may apply. Windows 7 SP1 and Windows 8.1 Update required.
Some editions are excluded: Windows 7 Enterprise, Windows 8/8.1 Enterprise, and Windows RT/RT 8.1. Active Software Assurance customers in volume licensing have the benefit to upgrade to Windows 10 Enterprise outside of this offer. We will be sharing more information and additional offer terms in coming months.
So, it clearly mentions: Windows RT/RT 8.1 is one of the excluded editions.
About the free Windows 10 free upgrade offer, here are more details.