Go to the console by pressing ~.

Now use any of these to alter the in-game stats:

Enter the number you want where the code says “#”.

Code - Effect

hurtme # - Damages you for however much damage you put as the number.
maps - Lists all maps
sk_plr_dmg_357 # - Set Damage Value for .357 Magnum
sk_plr_dmg_crossbow # - Set Damage Value for Crossbow
sk_plr_dmg_crowbar # - Set Damage Value for Crowbar
sk_plr_dmg_grenade # - Set Damage Value for Grenades
sk_plr_dmg_pistol # - Set Damage Value for Pistol
sk_plr_dmg_ar2 # - Set Damage Value for Pulse Rifle
sk_plr_dmg_rpg_round # - Set Damage Value for RPG Rockets
sk_plr_dmg_buckshot # - Set Damage Value for Shotgun
sk_plr_dmg_smg1_grenade # - Set Damage Value for SMG Grenades
sk_plr_dmg_smg1 # - Set Damage Value for Submachine Gun
sk_max_357 # - Set Max Ammo for .357 Magnum
sk_max_crossbow # - Set Max Ammo for Crossbow
sk_max_grenade # - Set Max Ammo for hand Grenades
sk_max_pistol # - Set Max Ammo for Pistol
sk_max_ar2 # - Set Max Ammo for Pulse Rifle
sk_max_ar2_altfire # - Set Max Ammo for Pulse Rifle energy orbs
sk_max_rpg_round # - Set Max Ammo for RPG
sk_max_buckshot # - Set Max Ammo for Shotgun
sk_max_smg1_grenade # - Set Max Ammo for SMG Grenades
sk_max_smg1 # - Set Max Ammo for Submachine Gun