If it is raised a half an inch, it is not ring-worm (a fungal disease for those of you who are not familiar). It could be a bot fly (warble) infestation, a cyst, or several other things. It probably needs be lanced and drained, or, if it is a bot fly, the larva removed. It should then be medicated and kept clean till it heals. You need to consider how valuable the rabbit is to you. If it is very valuable, take it to the vet. If it is not so valuable, wrap it in a towel, have someone hold it, and lance the protuberance with a disinfected scalpel or single edge razor blade. You will then need a needle or a dental pick to pull out any offending material or larva. Clean with hydrogen peroxide, apply some tripple antibiotic, and you are done. Clean off the surface and re-apply the ointment twice daily. I usually keep a cup of generic Listerine mouthwash around to dip instruments in.
It may sound sort of yucky..but if you learn how to treat this one, you will know how to do it when it happens to a treasured animal.