That’s all stored in the AppleIIGS.xml.
i assume you are talking about the file path, if so under the tools menu option there should be a change rom path location option that should change them automatically.
FYI, I store my roms on an external drive as well, but I also virtual map that drive onto the LaunchBox/Games folder. That way paths are relative not absolute, and provides many benefits like not worrying about drive letter, easy replacement.