How to delete a unit

In Normal Mode

Select the unit you want to delete by left clicking within the unit area.
Press the Delete key,

Using Edit Modes

Enter Delete Mode (select Delete under the Tools menu, or click the Delete Mode toolbar icon ).
Left click within the area of the unit that you want to delete.
Acknowledge the confirmation dialog by clicking Ok.

Notes

The position of underlying units after a delete is affected by the current Insert Mode setting. If the Insert Mode is Insert then underlying units are shifted up. If the Insert Mode is Overwrite then only the thickness of the immediately underlying unit is changed, and other units further down the hole do not move.

When in Delete Mode you can change the Insert Mode by holding down the Shift key before clicking the unit that you want to delete. However, pressing Shift-Delete in Normal Mode does not change the Insert Mode for that action - this is an exception to the normal behaviour of the Shift key changing the Insert Mode.