Open the Cheat Box and enter modifyNeighborhoodTerrain on
This allows you to alter the neighbourhood terrain by raising or lowering it.Click over the area you would like to change.
To select a larger area, click and drag the cursor to highlight the desired area. Press [ or ] to raise or lower the terrain by one click and press \ to level the terrain.
To close the cheat,
Open the Cheat Box and enter modifyNeighborhoodTerrain off