If you look on line, there is some very good detailed advice on how to renovate an old overgrown apple tree.
The main principle is that you do a little each year, and nothing too drastic at one time.
Structural and major pruning should be done in the winter or early spring while the tree is dormant, but pruning of fruiting branches and general control and thinning can be done in July or earlier.
Research the matter thoroughly though before you start, as it's easy to wreck a tree by overdoing it, and you can't go back once it's been done.