This trainer perk functions like the wizard's familiar ability (part of the arcane bond class feature) except as noted here.

• The trainer’s effective wizard level is equal to his trainer level –3 (minimum 1)
• A trainer who selects a familiar can choose one monster he has captured that meets the requirements to be a familiar for his effective wizard level. While active, that monster becomes the trainer’s familiar.
  

When the monster trainer gains a new level in any class that gains a familiar, he can update his familiar or choose a new monster he has captured that meets the requirements to be a familiar for a wizard of his effective level.