This tutorial covers promoting and demoting users at the account level and the project level.
Things to Consider
Roles are changed from the actions menu at the end of a person's row, not from a separate edit screen.
An account role and a project role stack. Someone can be an Account Viewer across the portfolio and a Project Admin on one job.
Account Collaborators can make limited role changes. If a change you expect is not offered, you likely need Account Admin.
Change an account role
Open Settings (gear icon, bottom of the left sidebar) and click Users under Account.
Find the person. The Search box and the Account role filter help on long lists.
Click the actions menu at the end of their row.
Choose the role you want. The options name the destination directly, for example Promote to Account Admin, Demote to Account Collaborator, or Demote to Account Viewer. You will only see the moves that apply to their current role.
Change a project role
Open Settings and click All projects under Projects.
Click the project, and stay on the Users tab.
Use the same actions menu on their row.
Not sure which role to pick? See Account Roles and Project Roles Explained.