How to treat an allergic reaction to spermicide condom?

Avoid and meds. Most allergic reactions if mild can be treated with antihistamines. If the allergic reaction is more significant, then corticosteroids (creams or tabs) may be helpful. It would be useful to know if the reaction was from latex or the spermicide in an effort to prevent future reactions. An allergist can assist in this endeavor.
See a dermatologist. Since this is a contact reaction rather than a histamine reaction, no antihistamine will help you. A cortisone cream will help.