I would agree with Hawkwalk... however one can prune a healthy apple tree any time except fall. The pruning encourages new growth and, of course, the tree should be preparing for dormancy at that time. As mentioned, the best time is late winter. Be sure to use sharp pruning cutters so as to leave a clean cut. You've experienced one of the problems with not pruning a tree until it's well established. That being alternate bearing years... one year heavy, the next light. Best of luck!