I have bought a garden recently and It seems like these apple trees have never been pruned. I don't know If I should cut down main branches to make these apple trees something like open center or central leader. All branches are gone straight forward to sky. I would be so much grateful if someone show me where should i cut with red marks on picture.
Pruning a free-standing apple tree is much easier than pruning one trained as a cordon or espalier. Old gardening books recommended different pruning methods for different type of apple, but if you get the basics right you don't need to worry about that level of detail.
Start by removing all the weak looking growth, and any branches that are crossing or rubbing against each other.
You want to have 4 or 5 main branches forming a "bowl" shape and open in the middle. Your picture looks like the tree already has that basic shape.
The final step is to cut back about 1/2 to 1/3 of the length of last year's growth, just above a bud which is pointing in the direction you want the branch to grow in the next season.
DON'T trim off the short side shoots from the main branches, because they are where next year's fruit will form!
If the tree hasn't been pruned for several years, I would suggest not doing more than that in the first year. If the tree is wider or taller than you want, in future years you can cut out complete branches to reduce the size, and prune what is left in the same way as in the first year, but see how it responds to the first pruning before you do anything irreversible to it. The harder you prune the tree, the more it will grow the next year, so an attempt to make the tree a lot smaller in a single pruning can be counter-productive.