The newest GarageBand version is 10.0.1. However, this version is only available for OS X Mavericks, so you cannot install it.
The most recent GarageBand version for your Mac is 6.0.5, and it can be downloaded from the Apple website > http://support.apple.com/kb/dl1507 However, it only works with GarageBand versions installed from a DVD, and it looks like you got GarageBand with your Mac or from the Mac App Store, so you cannot install it. Only GarageBand 10.0.1 is available in the Mac App Store, so you have two options:
1. Stay with GarageBand 6.0.4, or;
2. Upgrade to OS X Mavericks from the Mac App Store and install GarageBand 10.0.1. Make a backup of your files with Time Machine and check that your apps are compatible > http://www.roaringapps.com