Deleting a test/ free user OR Reassign test user to Business account

Deleting a test/ free user OR Reassign test user to Business account

I have a test/ free user - now I would like to be added to our business account as an additional user on our subscription.

BUT - the system rejects my email address as "already in use".

How can I either delete my test user, OR reassign this user to our business account?

Thanks