Hello!
Try "[b]Interpolation and assignment[/b]" tool, you can find it under [b]Tools[/b]
press [b]next[/b] and select the layer you want to change, in your case [b]MOUSE Manholes[/b] and attribute would be [b]GroundLevel[/b].
press next and select "[b]Assign from raster cells[/b]" and then see if you can locate your DEM file in the listmenu, the first one of the two. If so, continue and check the second listmenu and select "[b]pixel value[/b]"
Continue and from there you have one more window with options, if i'd change all manholes i'd keep every box unselected.
