Replacing skeletal meshes with blender

Hi guys, new to modding and am looking for a tutorial (video or text) on how to replace existing skeletal meshes in blender. I an trying to modify the standard club skeletal mesh but am lost on how to do so.

I have exported the weapon in Ark Dev to an FBX, imported it into blender but cant seem to unbind the stock mesh skin and use my own. Am I going about this the wrong way or is there another free program out there that works better?

Any help would be appreciated! Thanks in advance