To retrain is to teach (or to learn) all new information and skills. Once your dog retires from guarding sheep, you can retrain her to be a visiting therapy dog in nursing homes.
When someone loses a job in one industry, a state employment agency can often help to retrain them for new kinds of work. A mill worker might retrain to be a computer programmer, for example. The verb retrain has the "again" prefix re- added to train, "to discipline or teach," a definition that grew out of the meaning "manipulate and shape," the way a gardener trains a vine to grow up an arbor.