How to save changes of fbx subassets ?

Hello

I am trying to change a mesh subasset of a fbx asset, when I use my script I am changing the triangels as I want and I can see the changes on the Scene view. But when I quit unity and oppening it again it doesnt save the changes.

I try using AssetDatabase.Refresh(ImportAssetOptions.ForceUpdate); but it didnt do the job

any idea what to do ?

Thanks :)