The best way to fertilize it is to ensure it is released, meaning not shaded by other trees. Next, would be to clear competing brush from around it. Third, prune dead, dying, or crossed branches. Fourth, open up the center by pruning vertical branches. After all that is done I may consider fertilizing, but the number of trees where fertilization is the issue are few and far between.
what i have done to revitalize apple trees is, 1. remove all brush from trunk to 10' beyone the drip-edge, 2. prune all water sprouts, (small upright branches that grow from the tree base of off the main trunk. 3. remove any and all branches dead, desaesed, or touching. 4. remove all branches growing toward the center toward the trunk (these will be shaded by the rest of the tree and recieve little nutrients and sunlight) soem peopkle recommend in some instances where the tree is so overgrown that a 3 yr prunign cycle should be done where 1/3rd of the tree be done each year as not to harm the tree to much. and do all your pruning before the buds start to show.
5. also i will take a spudbar and walk around the drip edge of the tree driving 6"-9"d holes every 2'-3' i will then fill these up with fertilizer (10 10 10 will do just fine) fertilizing in this method will allow a slow controled nutreint relases most of the year. secondly and this isnt a must but on old trees i will spread a light (2-3 cups worth) of borax along the drip edge, this will add borate which will slow downt the browning process when you cut an apple. hope this helps. the last 6-7 years i've made it a hobby of mine in bring old abandoned trees back to production. that all started with my tree 5 years ago, when i baught the house i thought it was a bush of some sorts, last year i got 13 gal of apples and all about the size a little smaller than a softball but bigger than a baseball and all were delicious. best guess is it is a stamin winesap.
A brief search will provide many sites with videos and graphics on pruning advice. Fertilizer is rarely needed, and in many cases is counter-productive while progressing with multi-year pruning renovation of older neglected apple trees.
Good luck, it is fun to watch the improvement in quality, but my experience is that it is hard to make myself remove enough wood to bring a tree to what orchardists would consider optimum for best market quality fruit.