From the People Planner > identify the person
Click on the 3-dot icon next to the person's name > select Edit Details
From the side panel, click on Contracts tab
Under Work Days, select Custom
De-select one day from Monday to Friday (if the person is off on every other Friday, take off Friday)
Click Save > select Update existing contract > Save again
From the People Planner, a rostered day off has been added every Friday
Right-click on the rostered day-offs you want to delete and click Delete
Once a rostered day off is deleted, it cannot be manually added back.