I have bleached out the yellowing by placing the decal sheet in a ziplock baggie and taping it decal side out to the inside of a south facing window for a few weeks (the baggie is just to keep any condensation that may form on the inside of the window from damaging the decals), Since you already have applied the decals to the model, you could try putting the entire model in a location where it will get direct sunlight to see if that will bleach out the yellowing. I've heard from other people who tried this trick that, in some cases, the yellowing eventually returned. Good luck.