This is a collection of official firmwares, which can be used to upgrade or downgrade the Media Nav.

Versions 1.x.x – 4.x.x: Media Nav (MN1) Firmwares
Versions 5.x.x – 9.x.x: Media Nav Evolution (MN2) Firmwares
Versions 1.x.x.x – 3.x.x.x: Media Nav Evolution 2 (MN3) Firmwares
Versions 6.x.x.x: Media Nav 4 (MN4) Firmwares

For a step-by-step guide on how to upgrade or downgrade the firmware, check out the Tutorials section:
How to upgrade the firmware
How to downgrade the firmware

Also, the source code for the firmwares is freely available, courtesy of LG Electronics – see Firmwares Source Code.