Go to the main menu, pick Extras and go to the Cheats screen where you can press one of the following codes:
Game Finished as Abbey: X, R, X, Y, X, Y, R, L
Game Finished as Conner: X, Y, L, Y, X, Y, L, R
Construction & Robots: Y, Y, Y, Y, Z, A, R
Dr. K's Base & Robots: Z, A, Y, Y, Y, X, A
Bubble & Town & Night & Robots: Y, L, Y, Y, X, L, Z
Bubble & Town & Day & Robots: Y, Y, Z, Y, Y, R, L