To edit your terrain, select the terrain topic from the left. Terrain points show the editable areas of the terrain as dots. To edit one single point select the point with the “Select” tool. To edit multiple dots select the “Select Area” tool or hold down command to multi select specific dots one by one. You can change the height of select dots by editing them from the arrow in 3D or by editing the value exactly from the parameter input on the right side.
By default your buildings are transparent in the terrain step, but you can also switch them to the facade mode to make sure your terrain modification are in line with your buildings facade.