If you already have a PC or tablet that came with Windows RT pre-installed, then you can get the free update to Windows RT 8.1 from the Windows Store. You can only install Windows RT 8.1 on PCs or tablets that are already running Windows RT.
https://support.microsoft.com/en-us/help/18521/windows-rt-faq
On a Windows RT device, you’ll need to install Modern apps from the Windows Store instead of desktop apps from the web. However, intrepid hackers — in the good sense of the word — have been hacking away at Microsoft’s limitations.
http://www.makeuseof.com/tag/how-to-jailbreak-your-windows-rt-device-and-run-unapproved-desktop-software/
Type windows update in the search box. Next, he had me click Check for updates. To my surprise, this Windows Classic view found a firmware update that the Modern UI update from the Charms menu missed. Once the updates were applied and the RT had rebooted, he had me check again.
http://www.techrepublic.com/blog/tablets-in-the-enterprise/fix-found-for-windows-81-upgrade-problems-on-surface-rt/
Fix found for Windows 8.1 upgrade problems on Surface RT. Donovan Colbert couldn't find the Windows Update to 8.1 in the Microsoft Store on his Surface RT tablet, so he turned to Microsoft support for help. Note: Microsoft temporarily halted and then resumed Windows update 8.1 for Surface RT since this article was written.
http://www.techrepublic.com/blog/tablets-in-the-enterprise/fix-found-for-windows-81-upgrade-problems-on-surface-rt/
How to Jailbreak Your Windows RT Device and Run Unapproved Desktop Software. Windows RT is the locked-down version of Windows 8 for ARM computers, as seen on Microsoft’s Surface RT tablet and a few other Windows RT devices. Unlike the standard version of Windows 8, Windows RT doesn’t allow you to install your own desktop programs. You’re limited to the included desktop programs written by Microsoft or Modern apps from the Windows Store. However, there are ways to jailbreak your Windows RT device and run unapproved software on the desktop.
http://www.makeuseof.com/tag/how-to-jailbreak-your-windows-rt-device-and-run-unapproved-desktop-software/
There are many ways that you can personalize your Windows RT 8.1 PC. You can customize the Start screen, change colors, set up a picture password, add languages, sync favorites and settings between PCs, and bring all of your friends, contacts, email, and photos together.
https://support.microsoft.com/en-us/help/18521/windows-rt-faq
Note that jailbreaking your Windows RT device won’t allow you to run any Windows desktop program. Windows RT devices run on ARM chips, while standard Windows PCs use Intel x86 chips — that means you can’t just run standard Windows software on an ARM device, even after jailbreaking it.
http://www.makeuseof.com/tag/how-to-jailbreak-your-windows-rt-device-and-run-unapproved-desktop-software/
Windows RT 8.1 contains many of the same features as Windows 8.1, but is designed for thin and light PCs that use ARM processors. Some of the features of both Windows 8.1 and Windows RT 8.1 are: Fluid, intuitive, and easy-to-use interface design that you can easily customize.
https://support.microsoft.com/en-us/help/18521/windows-rt-faq
If you don't mind getting a black Touch Cover, you can save yourself $20 by picking up a $600 Surface RT with 32GB storage and black Touch Cover. Other Touch Cover colors include white, magenta, cyan, or red. Type Cover appears to be black only.
http://www.pcworld.com/article/2012126/microsoft-surface-with-windows-rt-5-things-to-consider-before-buying.html
With Windows RT 8.1, you can install apps directly from the Windows Store, but you can't install desktop apps that you used with previous versions of Windows.You can only install printers, mice, keyboards, and other devices that have the Designed for Windows RT logo. For more info, see the Windows RT disclaimer.
https://support.microsoft.com/en-us/help/18521/windows-rt-faq
what type of software can be install on my surface rt
Windows 8 is a type of software that can be installed on  Surface RT.