Demorph (or demorphing) refers to the process of exiting a morph and returning to one's natural body. If one wishes to change morphs, one must demorph before doing so and then morph from one's natural self. It is, for the most part, impossible to demorph after one has spent more than two Earth hours in a morph, although there have been exceptions.
In the first few books, the term demorphing was not used. Instead, the characters would say "morph out/back" or "unmorph". For example, in #4 The Message, Jake tells Marco to morph back to human after he is bitten by a shark in dolphin morph.