well, my apartment is an upstairs apartment, and I have neighbors below me. The first thing I would do is stock up on supplies, get some hand held games, a generator, and the necessities.
I’d fill my bathtub, and sinks up with water (eventually the water will cut out). I’d be sure to have some semi automatic pistols, and rifles with plenty of ammunition. My melee weapon of choice would be a crow bar (if I needed, I can pry open doors that are locked if on the move, and it won’t ever break by beating in zombie skulls).
After I’m fully stocked, which may take some time, I will use a sledge hammer to destroy the stairs leading up to my apartment, and to be sure I’d have a rope I could use to climb back down if needed.
I’d be fully stocked up, armed to kill zombies, and they would be unable to make it up to my apartment since I have destroyed access by taking down the stairs.
I would hold out as long as possible, and when I feel it to be safe, I’d make my way to a marina of some sort, and try and make my way out to some sort of oil rig out in the ocean and focus on just surviving out in the elements, fishing and things like that, maybe eventually locate an island, live off the land and so on.
Assuming it got that bad, I suppose I could hold out for at least a couple months if I ration my food appropriately.