Log in to Digiposte with your login email and password https://secure.digiposte.fr/identification-plus 

Go to your Profile at the top right of the page > Account & Preferences > , in the Email Address field, click Edit:

Depending on the background of your e-mail with La Poste, there are three possible actions: 

Scenario 1: La Poste does not recognise the email address: The change is applied after confirming your e-mail.

Scenario 2: The address is already linked to a La Poste account (without a Digiposte safe): Retrieve the password for your La Poste account to link it to your Digiposte safe.

Scenario 3: There is already a Digiposte safe linked to this email : You can merge the two safes.