I'm trying to retexture the Advanced Combat Armor Mark II, but I can't find the texture do edit on texture sets; in addition to that I don't know how to download the file once I do find it.

Could somebody provide me with the name of texture I'm looking for and a way to download the textures in New Vegas?

You'll want to extract the NIF, open it up in NifSkope and see what the texture path points to. You can then extract that texture/others from the BSA.

How do I see the texturepath?

Look at all the entries at the bottom of NifScope. You'll need to expand some of them. Somewhere in there you'll see one that deals with the object's texture.