When I want to use Proposed Topography > Window Delete or Polygon Delete, maybe I don't understand how it works, but I try to delete some points in the topography, but it didn't work.
Window delete you need to click and drag. Then a dialog box will ask you if you are sure you want to delete. Polygon delete you click around the area you want to delete then double click to close the polygon. Then a dialog box will ask you if you are sure you want to delete.
Make sure you have the Proposed Topography ticked on the in the Project Explorer.
Note if you want to delete points from the Existing Topography then you would go Tools>Existing Topography>Window Delete or Polygon Delete.