I imported an .fbx file from blender to Unity and it is shown as a prefab but with slightly different icon. Is it really a prefab? Why I can't edit any of its properties? Also, how do I change its mesh if I update the source model? Seems like I can't do anything with it:
Imported fbx files are converted to immutable prefabs during the import process. Depending on what you need to change you have to do so in Blender.
If you want to have an editable version (ie. one you can add scripts to or change the material on or whatnot), you can just create a new prefab by dragging the model into the scene, and then dragging that back into the project view. You probably want to create a prefab variant, that will allow you to add more meshes in blender and have that be added to the prefab.