To transfer the ownership of the account to another user for legacy Essential Plans (Free, Starter & Team Plan):
Only an account admin can grant account admin role to other users.
Invite the new user to every single guideline, project, and library on the account with owner role.
Grant the new person the account admin role by following the instructions in the article here: Change of the Account Administrator
Once this is done, the new account admin can remove the admin rights of the old admin and remove that user from all the projects, if the user does not need access anymore.
Update your billing information if they changed. Learn more about that here: Update Billing information.
If you are on an Enterprise Plan, please contact your Customer Success Manager.