A person who managed a project has left, how do I remove them from the project?

Project > Project Members > #project member

The Administrator can remove the name of the individual from the Project and assign a new Project Administrator to this role. 

or

Account > Members > Project 

To add a new person to the Project, you can do so from either of these two areas.

Project > Project Members > 3project member > +Invite Account/Project Member

Account > Members > Account/Project > +Invite Account/Project Member

Click the drop-down arrow and choose which role the new member will have.