when you go to export the model, you should click the top option, this will give you a .json file, save it as whatever it is youre replacing in game. then youll want to open up that file and at the top there will be the directory to the texture used, you should replace the bits before the '/' with 'minecraft:item', like adding a new texture it will need its own file branch thingy, minecraft > models > item, put the model in there and the texture in minecraft > textures > item

Note that only the .json file will need to be named what its replacing, the texture ca be whatever you like as long as its referenced correctly

If youve any issues you can always download a pack that changes a similar thing and see how its all set up