Download mb2md.pl script and run as described in manual.
For example to convert /old/mbox to /new/maildir you can execute:
# perl ./mb2md.pl -s /old/mbox -d /new/maildir
Converting /old/mbox to maildir: /new/maildir
Source Mbox is /old/mbox
Target Maildir is /new/maildir
Dummy mail system first message detected and not saved.
40 messages.