Heh, I totally misread your post initially - sorry if you managed to read it in the few seconds before I edited it.
If all you're looking to do is reset Rockbox, you can do this via the "Tools" menu in emCORE:
Tools - Clear Rockbox configuration
If you are wanting to mount and update Rockbox with
a more recent binary*, you can (probably {it can be flakey}) mount the under Rockbox by booting the fallback image - again from the emCORE "Tools" menu:
Tools - Run Rockbox fallback image
[Saint]
*This link will always point to the most current version of Rockbox for your device - the build that comes with the installer is ancient - many improvements have been made since then.