You'd need to have access to your Tableau Portal Account, and I think you need to be an admin of the account (unrelated to a Tableau Server Admin). From there you can reassign your Key(s) to other users.
Let me know if this helps!
Tableau Online does not have license keys like the Tableau Server, Desktop, and Prep products do, and the seats you have purchased can only be managed from within your Tableau Online site as opposed to the Customer Portal.
If you only have 1 Tableau Online seat, you will not be able to reassign the seat to a new user by yourself. Please open up a support case and we will direct your issue to the correct team.
Senior Technical Lead
Tableau Server & Desktop Certified Associate