Upon reaching 4th level, and at every even-numbered sorcerer level after that (6th, 8th, and so on), a sorcerer can choose to learn a new spell in place of one she already knows. In effect, the sorcerer loses the old spell in exchange for the new one. The new spell's level must be the same as that of the spell being exchanged. A sorcerer may swap only a single spell at any given level, and must choose whether or not to swap the spell at the same time that she gains new spells known for the level.
Since all new spells are learned at odd levels how can a Sorcerer exhange a spell under the rules above?