If the series is in your scheduled recording list, you can select any upcoming recording, hit the REC button (or select the red dot). Then select "Modify Recording settings" (triple dot icon). Then finally select "Don't record this series." NOTE: The option to "Not record this program" on the first screen will only cancel that selected scheduled recording, not the series.
If the series is not listed, you can go to "Manage My DVR" from the DVR menu, then "Series Priority". Use the up-down arrows (don't use PageUp/PageDn, as they MOVE a show up or down in priority) to find your show. Hit OK/Select to choose that show, then the "X" icon to cancel the series.