You can use rekordbox ver. 5 by following these steps:
Windows:
Install ver. 5 again.
Double-click the rekordbox icon on the desktop.
Mac:
Use Finder to select [Applications], then click the [rekordbox 5] folder.
Double-click rekordbox. app.
Your ver. 5 library will be unchanged since you converted it for use with ver. 6. New cue information or tracks you've imported to ver. 6 won't be included in your ver. 5 library.