Go into your csgo folder, in the csgo folder click on scripts and item. In there should be a notepad file. What you do now is make a copy of the original file and put the original file on the desktop and keep the copied file there. Open the file and there be this huge list of um things. Click on Control + F and in that box enter in "weapon_knife' and click search 3 times. After you are at the location go to model_player and you should see something like models/weapon/default. Delete that line and um use this List and replace it with the knife you want. Do the same for Model_World. When you copy and paste the thing under here make sure to delete the v in weapons/v and replace it with w so it should look like this/w . That is only for the Model_World one
i will make a guide on this for the knives
Bayonet - models/weapons/v_knife_bayonet.mdl
Flip Knife - models/weapons/v_knife_flip.mdl
Gut Knife - models/weapons/v_knife_gut.mdl
Karambit - models/weapons/v_knife_karam.mdl
M9 Bayonet - models/weapons/v_knife_m9_bay.mdl
Huntsman Knife - models/weapons/v_knife_tactical.mdl
Butterfly Knife - models/weapons/v_knife_butterfly.mdl
Falchion Knife - models/weapons/v_knife_falchion_advanced.mdl
Shadow Daggers - models/weapons/v_knife_push.mdl
Bowie Knife - models/weapons/v_knife_survival_bowie.mdl