Apple trees, like all other trees, tend to keep growing taller as they get older, and even semi-dwarf trees can easily reach 15 to 20 feet tall. Pruning the highest branches allows light to penetrate farther into the center of the tree, keeping the lower branches healthy, and keeps the fruit ripening at a manageable height. Older trees that have been neglected and left unpruned for more than a few years will need extra thinning to rejuvenate them and encourage branches to grow in the lower third of the tree.
Remove any dead or diseased wood wherever you find it. Use pruning shears for 1/4-inch shoots, loppers for 1/2-inch branches, and a pruning saw for any branch over 1 inch in diameter. If the shoots are diseased, cut at the base and avoid cutting into any obvious cankers. Dip your tool in Clorox to sterilize it after pruning.
Cut out the largest branches that need to be removed. Work from the center of the tree outward, assessing the framework of the tree and taking out any limbs that are pointing back toward the center and any that are crossing another branch.
Remove the next smaller branches, still working outward. Cut a few shoots out of any area that seems congested to improve air circulation.
Take the tallest branches off if they extend beyond the desired height of the tree. Don't cut them off in the middle; follow them down below the shorter branches until you find the place where they join a larger branch and remove them at that point. This way, you're shrinking the tree without leaving stubs to branch out in all directions.