How do you reverse allergic reaction to penicillin?

This should in no way be considered "medical advise" or a specific course of treatment.
Typically serious penicillin reactions require IMMEDIATE EMERGENCY TREATMENT with an injection of epinephrine, supportive oxygen, and possibly a steroid injection, possibly an injection of an antihistamine, and careful monitoring of the patients airway. If the patients throat begins to swell shut, consideration should be given to intubation (putting in a breathing tube). If the airway continues to swell, intubation may no longer be possible (we may not be able to get the breathing tube in), which would result in a tracheotomy.

Rarely are reactions this serious, however it is extremely important to get immediate medical treatment regardless. Sometimes reaction can go from somewhat mild to severe very quickly.

If you have a mild allergic reaction, you may be able to control your symptoms with over-the-counter antihistamines or other prescription medication which is available from your doctor.

If you have previously had a serious reaction to penicillin, you may wish to consider carrying an allergy kit of sorts, which contains a shot of epinephrine.