Get hold of a program like ultraISO or superiso and just extract the navboot file, then mod, then replace/overwrite, then write, then burn (using imgburn).

As scuba says, you will need to downgrade then re-upgrade, you should of done the mods before you updated really.

HTH

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.