Depends on the situation:
- are you targeting a specific location to maximize the effects of your attack (i.e. targeting an area with no or less armor, a vital point, the only vulnerable part of a monster, etc)?
No change, save maybe in the description of the attack. The rules already assume that your character is trying to hit his foe where he's most vulnerable. If you end up hitting him in another location, it's because he partially deflected your blow.
- are you trying to achieve something specific (i.e. hitting him on his arm to make him drop something; trying to blind him; knock him out;...)?
Use the appropriate Combat Maneuver (disarm, swashbuckling,...) or Ambush.