Open your Skyrim directory in steam (it's in a folder in steam called commonapps or something like that), go to models, find insanity's folder with weapons, and find the nif file that's likely called the same or very similar as in the game (probably daedric sword or eastern daedric sword or something in those lines, I don't have that particular Insanity's sword in my game so I can't check). Copy the file into weapon's meshes folder. It should be in meshes\weapons\daedric\ and rename the file to daedricsword.nif which should automatically replace the regular, one handed daedric sword into Insanity's one.
Let me know if this is too confusing, and I'll try to go more step by step, I understand that a lot of people likely never even opened Skyrim's directory more than once or twice if they use a mod manager or something.