I think I have erythema multiforme. What is the cure?

Depends. Erythema multiforme is a rash that is the final common pathway of many different conditions - often allergic type reactions to virus or medication. Treatment varies depending on what is causing the rash. This rash is often confused with urticaria which has similar causes. I would not make this DX without consulting your md since some cases of em can progress and become severe.