You can edit your default address per user by going to Settings. To edit your default sender address:
Click the Settings tab.
Under Admin Settings select User Management.
Click the edit icon next to the user you wish to set the default sender address for.
A new page will open with the details for this user. From here, update the address, click Save Changes and refresh your browser; the sender address will be updated.
For more details visit the Help Center article: User Management.