If you have a Microsoft Surface RT or Surface 2, the standard way to "reinstall" the OS is through a Recovery Image Surface Recovery Image Download - Microsoft Support

However, you can still restore your device using the following methods: 1. Official Surface Recovery Image

no official standalone ISO download for Windows RT 8.1 . Because Windows RT is an ARM-based operating system pre-installed by manufacturers, Microsoft does not provide it as a separate software package for manual installation from external sources like USB or DVD. Eight Forums