To edit parameters in multiple books simultaneously:
1. Select the needed books (using Ctrl, Shift or Ctrl+A)
2. Open Books -> Edit Books menu
3. Check the box near each parameter you want to edit and select the needed value
4. Press OK
Note, that for Genres, Authors and Tags you can either replace the existing values, or add the new values to them. If you want to clear e.g. Tags - leave it empty and select the "Replace current tags" option.