Pruning apple trees normally begins immediately after planting. Remove the central stem to just above the highest side branch. For the following 3 years, prune only the tips of the remaining main branches by one third in winter. Aim for about six main branches which will form the frame of your tree, with fruiting sub branches growing off of them. From the fourth year, some sub branches can be pruned out at the union where they join the main branch, to allow new sub branches to take their place.
However, if dealing with overcrowded mature trees then do some renovation pruning to restore the tree to a nice open shape and improve air circulation and light penetration. But be careful not to remove more than 25% of the canopy per year. This type of pruning should also be undertaken in winter. First remove all damaged, dead, crossing and diseased branches. Then remove any crowded branches to create a nice, even framework. Once the unwanted branches have been thinned out, begin a regular pruning regime, removing older fruiting spurs so that younger ones can replace them. This renovation process needs to spread over several years though so will require a bit of patience and may also reduce the crop initially