so to make the textures changeable once you have imported your model into unreal engine editor open it up to apply materials , then just below where you pick the material itself it will say slot name change this to one of the following:
panel_#
glass_#
opaqueglass_#
trim_#
grill_#
this should then be followed by a number , any number

Then export as normal and in game it should allow you to change the materials in game