By choosing a loaded transport, clicking the assigned button and choosing a spot of unload with right-click, you will order your air transport to release the carried unit on chosen spot. The command also has an area version with the following modifiers:
- By clicking and then dragging you can order multiple transport to unload units in a general area.
- By clicking, then dragging while pressing Alt+Ctrl you can order transports to unload units in a line.
- Transports decelerate and come down before unloading, rendering both them and the cargo vulnerable.
For more information on transportation see this.