Which version of GarageBand are you currently running?
If you are currently running GarageBand '11, version 6.0.5, you are having the last free update installed. GarageBand 10.0.3 is a new release and requires a purchase to upgrade to this new application. It will only come for free with a new Mac.
Before you decide to buy GarageBand 10, look at the Help page (GarageBand 10.0 Help online Learn how to use GarageBand with this online resource.), to make up your mind, if you will like it. It is very different from GarageBand '11. Some features are no longer supported. There is no longer a podcast track, there are no more Sound Effect instruments, that you can customize with samples, the movie track is more difficult to use, the vocal tracks have less features.
On the other hand you now have the midi draw area, you have smart instruments, like automatic drummers or arpeggiators, smart strings.
If you upgrade to GarageBand 10, don't uninstall GarageBand '11. It is hard to get back, and you may want to be able to use the GarageBAnd '11 instruments in GarageBand 10.0.
Both versions can coexist.