Place files in 

mods/update/x64/dlcpacks/patchday3ng/dlc.rpf/x64/models/cdimages/weapons.rpf

mods/update/x64/dlcpacks/patchday2ng/dlc.rpf/x64/models/cdimages/weapons.rpf

Replaces "w_me_knife_01"

*REMEMBER TO BACK UP YOUR FILES*