Currently Being ModeratedOct 4, 2013 3:31 PM (in response to Samuelphsr)
Have you moved your GarageBand application to a different folder and it is no longer in the applications folder? Then the update cannot succeeds, because the updater will not recognize the app if it has been moved or renamed.
Also, the update needs to be done as administrator. And firewalls and AV software should be disabled, also the energy saver turned off, while installing. all that might preventGarageBand from completing the update.
Here is a ling thread with possible fixes:
Try mteep's manual install
or if all fails, uninstall and trash the preferences, see Dagon's post: