Cause I just downloaded a mod, but the cuirasses and torso pieces are set to be equipped on the ankle. I want to change it so it takes a chest slot. How do I do this without breaking the mod and/or the game?

Open the model in NifSkope and find any NiTriShapes in the model called Tri Left Ankle 0, 1, 2, 3 etc. and change them to Tri Chest 0, 1, 2, 3 etc.

Then find the body part objects of the objects in the CS, and change it from an Ankle to a Chest object in the drop down menu.
Then find the armor object itself in the CS, and change it so it replaces the chest with your body piece.

I'm not using the CS atm, that was off the top of my head, but hopefully that's right (and if it isn't I'm sure someone will correct me ).