The login credentials for your Pickware account are also the ones you use to log in as the shop owner in your Pickware Admin. You can change them in the Pickware account using the button with your name at 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 have access to the Pickware Admin can be created and edited in the Pickware Admin under Settings → System → Users & Roles. You can find more information about this here.