Removing files from the CAB file probably won’t be a good idea. The installer will be looking for them and will probably throw an error if they aren’t there.
Updating the Access database should be easy. Double click on the CAB file, and copy out the file. Update it as needed and drag it back into the CAB file. Windows should be able to do this without issue. If it can’t winzip or winrar can.
As long as the file has the same name, and is in the same location the installer should work with it no problem.
As for removing the un-needed files, you’d need to contact the people that wrote the software and have them remove the files from the installer.