What is Behavior Modification?

Legal Definition
Behavior modification is based on methodological behaviorism, which refers to limiting behavior-change procedures to behaviors that are observable and was employed briefly during the late 1950s but predominately from the late 1970s to early 1980s. Specifically, behavior was modified through the use of presumed consequences, including positive and negative reinforcement contingencies to increase desirable behavior or by administering extinction and/or punishment to reduce behavior.

In contrast to behavior analysis, analyzing the behavior-environment interactions (including antecedent stimuli) was not considered relevant in behavior modification; it also lacked the conceptual piece (radical behaviorism) initially purposed by B. F. Skinner. Subsequently, applied behavior analysis (ABA) has superseded the early term behavior modification since the 1990s.
-- Wikipedia
Legal Definition
Treating disorders by replacing the undesired behavior with desirable ones. This is done with negative or positive reinforcement.