Navigate to the Users page, and click the kabob menu (3 vertical dots) next to the user who's role you'd like to change.
Select the role to assign (see the roles and permissions table below).
Note: Guests only have access to specific projects. If you are downgrading a user to a Guest role, you'll choose which projects this user may have access to.
Check the box to make this user a Billing Manager.
Billing Managers are free or paid, depending on the role they hold (eg, Editor or Commenter). They can add or remove users from the org, manage billing and add payment methods, and view usage reports. Orgs may designate more than one user to be a billing manager.
Here's a review of Reduct roles & permissions, and which seats are paid or free.