The use of the App Store, for Mac Apps (system upgrade and associated applications)
should be available to you after your computer had been running Snow Leopard 10.6.8.
There would be no additional "upgrade" beyond Snow Leopard 10.6.8 and its parts
from the built-in software update inside Snow Leopard itself; the next OS X is not
considered an update, beyond that, it would be an upgrade... Check the App store.
If your computer could run El Capitan, there is a newest version iTunes available as
part of that upgrade to OS X 10.11. If not, you may still be able to purchase Mt Lion
10.8.5 or Lion 10.7.5 (a download code, if still exists) should your MacBook be too old.
Good luck & happy computing! 🙂