It’s not that simple, depending on your connection type you might not be able to upgrade.
In smaller blocks they used the same tech as houses, FTTN, but in larger blocks they used FTTB.
In any buildings that they were going to do an upgrade, they have said they will charge the building $275 per unit, and will only do an entire building, not individual units.
I’m not sure if that applies to FTTN buildings but would seem likely as even though three’s no main connection into the building, they’d still have to work within strata property to run the fibre into each unit, so would make sense they would only agree to do all at the same time, and i’m guessing they charge the strata when organising it, you’d need to contact NBN to find out.
For FTTB buildings NBN currently hasn’t figured out what they’re going to do, they’re doing some trials, but in most cases you can’t currently upgrade.
Go to http://www.nbnco.com.au and put in your address, that will show you what connection type you have (FTTB or FTTN) and whether any upgrade is available. If it doesn’t mention upgrades available, there’s nothing you can do (apart from go with a different provider if available in your area).