If you have registered (and activated) using an incorrect onboarding code or email address then you will need to contact your school to deactivate your account. After that you will be able to register again using the correct onboarding code supplied by your school.

I have registered, but not yet activated

If you haven't activated your account by clicking on the link in the activation email, then you can still register again using your correct onboarding code. This will be supplied by your school.