Go to the Specials under the pause menu, then toggle them on or off.

Skin	How to unlock
Dan	Collect all Ttanium Bolts in single player.
Hiro Ratchet	2 Titanium Bolts.
Kanga-Ratchet	2 Titanium Bolts.
Plundering Pirate Captain Skin	6 Titanium Bolts.
Ratchetzilla Skin	6 Titanium Bolts.
Troical Vacation Skin	4 Titanium Bolts.