Generally, your best bet is to unregister the original COM, update the file and register the new one. I would advise you to rebuild the COM interop for the DLL. This is most likely easily accomplished by deleting the reference to the library and adding it back in. This will re-run the program that generates the RCW (runtime callable wrapper) interop assembly.
Technically, if the public interfaces on the classes haven't changed you might be able to get away with simply overwriting the DLL file with the update. However, this can be problematic.
Either way you do it, you can not have side-by-side execution of 2 COM libraries with the same name because of the way that COM works.