Your co-parent has changed their email address and you want to update it in the account?
If your co-parent has not yet accepted your invitation, you can change it in the "My family" area:

If your co-parent is active (has accepted your invitation), then you can't change it anymore. He has his own account. He has to log in with his old email address and change it in his "Profile", on the web version: https://tools.2houses.com/settings/profile/edit
Or on the mobile app, in his settings, next to his name and first name: