If you are the original owner of the account you are able to assign a new owner if you follow these steps:
Click on the "edit" of the agent or admin in the list of agents
Select "Owner can manage the account, billing and system settings, view and answer tickets" radio button and press "Save".
As you can see in the list of agents - both of the agents now have "owner" role access.
What to do when you would like to remove the original owner´s privileges and change their role to "admin" or agent"?
Navigate back to the list of agents and click on the "edit" of the original owner.
Select "Admin can view and answer tickets and manage system settings". The notification "You are trying to change the role of the account creator. Please select a new account creator first" should appear. You can set yourself in the options. After you´ve done so, you can click on "save".
Once you set the role of the agent you are able to continue with setup: