To change your email address after joining the Professional DJ team/Cloud Option DJ team, you need to unregister your Dropbox account from the team and change your email address in the settings page of your Dropbox account.
Follow the steps below:

Step 1: Select [Cancel setting] and remove your Dropbox account from the team via the Cloud Settings on My Page at rekordbox.com.
Step 2: Change the email address for your Dropbox account.
Step 3: Join the team using the Dropbox account again via the Cloud Settings on My Page at rekordbox.com.

Find out more about changing the email address for your Dropbox account here.