You can log in with your Codio username or the registered email address.

If you have also forgotten your password you can request a new password using either your username or email address.
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Resetting your password will send an email to your registered email addresss containing a link for you to click. This will then allow you to enter a new password of your choosing

See Forgot your password for more information. 

If you are a student in a Course set up by your teacher, they can also reset your password for you if you can't recall your username or email address. More details on how they do this can be found here: Change students password
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