To change the password of your mailbox, start by logging into gandi.net, and then go to your domain's control panel (click on the domain in your list of domains if you have several). Find this bit in the upper-right hand of the page:
Click on “Manage”.
On the “E-mail account management” list of mailboxes, click on the email address for which you want to change the password.
You will then see a form like the following, but with your email address indicated at the top:
Simply enter the new password, and click “Change” to submit.