Change the bond type of an existing bond

Now the oxygen atom is singly bonded to the carbon, we need to change this into a double bond.

Click on the bond between the carbon and the new oxygen and hold the mouse button down util a pop-up menu appears
(make sure you click only on the bond, not on an atom)
 
Select the Bond Popup → Type → Double command (see the picture)

The single bond has changed into a double bond.

Another way to modify a bond type is to click on the bond once which will select this bond. Then click on the bond tool in the menu bar ('ball and stick' logo), and select the proper bond type.

Yet another way to change the bond order is to use the keyboard shortcuts. If you have selected a bond, press 1 for a single bond, 2 for a double bond, 3 for a triple bond and 4 for an aromatic bond.

To get a reasonable geometry optimize the structure:

Press the pre-optimize button
If not converged, press pre-optimize again