You don't renew patents.
They're issued once, you get 20 years, and that's it. Then, the invention is in the public domain. That's the whole point of patents - legally protect an inventor from unauthorized use of the invention for 20 years, but then everyone gets to use the invention, not just people and companies to who, the inventor has granted a license.
There are ways to get around it - modify whatever it is very slightly, get a fresh patent, and then advertise the hell out of the new product (while slamming your own old product, because people are going to buy the now legal, and much cheaper, generic version of the old product instead of your version.) See: Claritin and Clarinex, Prilosec and Nexium.