All projects managed by a project manager can be edited under section Worktime > Administration > Persons. Select action Projects managed by person and select the person.
In Select projects you can search for projects and from the project hiearachy appearing under All Projects you can select single projects for which the person will be added as a project manager. The project manager rights for a project can be removed from the person by removing the project from the Selected projects list.
Confirm the changes by pressing Save.