Just stop taking it. No need to reduce dosage gradually or take weeks to come off it or anything.

But be aware that the body will just go back to the previously raised cholesterol levels very quickly. The Lipitor will not have altered that in any permanent way. The good effect of Lipitor stops as soon as you stop taking it. That may be fine if you want to try to control cholesterol by lifestyle changes instead but otherwise you will have the bad effects of raised cholesterol again.