If you wish to restore the file /cdrom/gtktalog/mycatalog.2 and your main catalog file is /home/you/mycatalog.1, you can do this two ways
Make sure GTKtalog is closed.
Copy /cdrom/gtktalog/mycatalog.2 to /home/you/mycatalog.2 ... and now use mycatalog.2 instead of mycatalog.1
Move the file /home/you/mycatalog.1 into /home/you/mycatalog.old and then copy /cdrom/gtktalog/mycatalog.2 into /home/you/mycatalog.1
Use your favorite file manager to carry out this copying.
Then relaunch GTKtalog