First go to the main menu of the game, go to extras, go to cheats, enter the secret skill to change the costume:
To unlock the costumes of the early 21st century please enter: CEILINGFAN
To unlock the costumes of the 90s please enter: WOODCHIPPER
To unlock the costumes of the 80s please enter: NAILGUN
To unlock all costumes please enter: V PIRATE