Get raw_mesh from UStaticMesh failed

I’m tryin to modify the raw_mesh of UStaticMesh, it woks well for most built-in staticmesh, but for the ‘sphere’ and the staticmesh imported from .obj, it failed just because get_raw_mesh return null, how can I set the property so that I can modify the raw_mesh, or Is there any other way to modify the mesh positions in UE?