How is red oxide of copper converted into black oxide & vice versa?
Red Copper oxide is Copper(I) oxide Cu2O. Black Copper oxide is Copper(II) oxide CuO. Copper(I) oxide is oxidised to Copper(II) oxide in moist air. 

Copper(I) oxide is oxidised to Cu(II) by dissolving in dilute nitric acid which can be thermally decomposed to Copper (II) oxide by heating at 180C.

Copper(II) oxide can be reduced back to Copper metal by Hydrogen, Carbon Monoxide or Carbon. This may then be heated with oxygen to make Copper(I) oxide. Copper(I) oxide may also be made by reaction of solutions of Copper(II) salts with Sulfur Dioxide. 

Source - Wikipedia entries for Copper(I) oxide Cu2O and Copper(II) oxide CuO