The part you would change is what
smok calls the "atomizer core". (More commonly called a "coil head").
When you need a new one, you just unscrew the old one, and replace it. Replacement coil heads are usually pretty cheap. I don't know the specific type of coil used on the cloud; but perhaps someone else can recommend a good place to get new ones.
However, a coil head should last a while, and if they start tasting bad you can try to wash it out with either water, vodka, etc. Eventually, though they will need to be replaced.