The login data for your Pickware account is also the login data for logging in as the shop owner in your Pickware Admin. You can adjust it in your Pickware account via the button with your name in the bottom left. There you can change your name, preferred language, email address, and password. You can also delete your account there.
All other user accounts that should get access to the Pickware Admin can be created and edited in the Pickware Admin under Settings → System → Users & Roles (Benutzer & Rollen). You can find more information on this here.