get the sdcard out of GameShell
download the image
https://mega.nz/#F!IUkxQSjB!d_H9leVSSl6tDP4X0oWnag
insert SD card into PC
use etcher or win32DiskImage ( if you are on windows ), I think etcher is easier to use
- select the downloaded image as source
- the inserted sd card as target
- flash it
- when it done, unplug sd-card from PC , re-insert into GameShell
took a etcher screenshot from @javelinface