You are able to do this from within the Project or you can go to Projects from the Navigation Bar to view the Projects. Once viewing the project, Go to the Properties card of the Project Module at the top of the screen, you will see the Project Manager listed (Under the Project Name). You should be able to modify it there by selecting a different user to be the Project Manager. If you are unable to modify it (lock symbol is seen), this means that your user does not have sufficient permissions to make this change. You need to be a Super User (Global Permissions).