How to change the default DKIM selector

The command line command was mentioned earlier in this thread:

You need to be in the /home/user-data/mail/dkim directory when you run this command.

root@box:/home/user-data/mail/dkim# ls
mail.private  mail.txt
root@box:/home/user-data/mail/dkim# sudo opendkim-genkey --bits=1024 -s box -d mailinabox.info
root@box:/home/user-data/mail/dkim# ls
box.private  box.txt  mail.private  mail.txt