Select one or more buildings, trees, or pieces of city furniture to display the transform gizmos.
Place your cursor on the Height gizmo.
Click and drag the cursor upward.
Watch the readout and release the mouse button when the desired height is reached.
For buildings, the height is increased or decreased while the footprint remains unchanged. For trees and city furniture, the object is scaled proportionately to the height you wanted.
Place your cursor on the Elevation gizmo above the Control Point to change.
Click and drag the cursor upward or downward.
Watch the readout and release the mouse button when the desired elevation is reached.
The control point is moved along the Z axis and the terrain under the selected object is also adjusted so there are no gaps.