Customizing a pre-designed 3D STL File

Does anyone know how to take a 3D STL file of a design that you can add images to?
I am trying to make some 1911 gun grips and I got a free 3D STL File and want to customize it by adding a family crest to the surface. I M NEW TO 3D STL files and have no idea how to do this if its possible. Anyone?

You most likely will have to open the stl in a 3d cad program in order to modify it. Another possibility might be to run two files, one cutting out the grip, and the other engraving the crest into it. Easel pro may be needed to do the stl.