• Start a private match
  
• Open the console (if you haven't bind it, you can do it in the ingame settings)
  
• Enter "sv_cheats true" in the console
  
• Enter "mp_drop_knife_enable true" in the console

• DON'T DROP YOUR KNIFE YET!
  
• Open Console and enter "give weapon_knife"
  
• Walk backwards and look at the new spawned knife on the ground (DO NOT PICK IT UP!)
  
• Enter the command "subclass_change 508" while you look at the knife
  
• Drop your current knife and pick up the new one

Change the ID number at the end of the"subclass_change {number / ID}" to use one of the knifes below . . .

500 - Bayonet
503 - Classic Knife
505 - Flip Knife
506 - Gut Knife
507 - Karambit
508 - M9 Bayonet
509 - Huntsman Knife
512 - Falchion Knife
514 - Bowie Knife
515 - Butterfly Knife
516 - Shadow Daggers
517 - Paracord Knife
518 - Survival Knife
519 - Ursus Knife
520 - Navaja Knife
521 - Nomad Knife
522 - Stiletto Knife
523 - Talon Knife
524 - Default Knife
525 - Skeleton Knife
526 - Kukri Knife

This only works in private matches with sv_cheats true, the knifes won't be added to your inventory!