This is in three parts. Firstly the factory OS needs to be reloaded and then the sound banks will have to be reloaded.
Finally, the global data will need to be reloaded. This will need to be with MIDI files and a sequencer as the memory for the KS is entirely flash based, there is no restore from ROM option. Please note that it is unlikely that the factory OS should ever need to be reloaded. Any problems which require a "reset" can usually be overcome by reloading the global data and/or the sound banks. The latest OS, sound banks and global data can be downloaded from the KS Rack downloads page. Full installation instructions are provided as part of the OS 2.1.02 download.