Other possibilities. A swollen glans (tip of penis) could also be due to balanitis (inflammation of the glans) related to fungal or bacterial infection, or other causes. It is important to be seen by a medical professional for proper diagnosis, as treatment can prevent complications (eg phimosis; a narrowing of the foreskin). Also, if fungal, evaluation for diabetes is worthwhile.
Penis swelling. If you feel pretty sure that it is an allergic reaction, the first step is to take an antihistamine like Benadryl (diphenhydramine) (otc). If the swelling is severe or painful, see a doctor; a prescription for cortisone may help. Allergic swelling usually itches, but does not hurt. If there is significant pain, then other causes need to be considered.