how to stop taking fiorinal?

Fiorinal contains Aspirin, Butabital and Caffeine. It is prescribed for tension headaches.

This is from http://www.drugs.com/cons/butalbital-and-aspirin.html

"When you use butalbital for a long time, your body may get used to it so that larger amounts are needed to produce the same effects. This is called tolerance to the medicine. Also, butalbital may become habit-forming (causing mental or physical dependence) when it is used for a long time or in large doses. Physical dependence may lead to withdrawal side effects when you stop taking the medicine. In patients who get headaches, the first symptom of withdrawal may be new (rebound) headaches.
If you have been taking large amounts of this medicine, or if you have been taking it egularly for several weeks or more, do not suddenly stop using it without first checking with your doctor . Your doctor may want you to reduce gradually the amount you are taking before stopping completely, to lessen the chance of withdrawal side effects."

You should talk to your doctor to work out a reducing dosage schedule so that you don't feel withdrawal symptoms.