To update to GarageBand for Mac (v10.0), install it from the Mac App Store. It is a new application and will not be in the Updates tab.

Existing sounds and loops will not be overwritten. Previous versions of GarageBand are placed in a sub-folder (named with the app version) in the Applications folder. For example, GarageBand 6.0.5 would placed in a folder of the same name. Both GarageBand for Mac (v10.0) and GarageBand 6.0.5 will be available under the Purchases tab in the Mac App Store if you want to reinstall.