Truthfully your best project on this bike would be to just go over all the major parts of the bike and ensure that they are clean, lubed correctly and adjusted spot on in every case. Any replacement parts you buy should be limited only to making the bike fit you better so you can ride in more comfort. The Yorkville is a good enough bike for what it is but nothing you do to it will make it into a lightweight racer. It is what it is.

Meanwhile the money you save by not doing a lot of upgrades can go into buying a nicer and lighter bike. Use the Yorkville along with test rides on some other bikes to fine tune what you want in a bike with the realities of how and where you ride firmly in mind. With a better bike will come the lighter weight you are after to aid in schlepping the bike up and down those stairs more easily.