Add new (bonded) atoms

Now, to revert to the ethanol molecule, we first remove the new doubly-bonded oxygen atom, and then add one hydrogen atom.

Click once on the doubly-bonded oxygen atom to select it
Press the backspace key to delete it

Select the H-tool
Click once on the carbon atom connected to the oxygen

Note that this way you started bonding mode again, as indicated by the bond to the mouse position.

Click once in empty space to make a hydrogen atom (connected to the carbon!)
Click once on the just created atom to stop bonding

Repeat this to add a second hydrogen to the carbon atom

Pre-optimize the molecule

Once again you have created an ethanol molecule.