Answer:
In order to retrain the horse not to buck with fear to a raised hand, the new owner shall counter condition the horse. Every time he has to pat the horse, he shall raise his hand so that the horse realizes the new condition that every time a hand will be raised, it will mean that his owner is patting him. Even when the new owner provides the horse with food, he shall raise his hand first while providing food so that the horse does not back off.