It says I have to update the software but there isn't an update for garageband on the App store.
Not everybody uses Garageband's wide spectrum of pre-recorded audio clips to use in their own mix. As they take up a lot of space on your harddrive, the average user will be very happy to save that extra space.
People who require more loops can download them via the Software Update:
This is the normal procedure of getting the free, extra audio clips.
Wat you are experiencing could be an error. Are you seeing any error message that you could add to your question? This would give us more information to help you. Otherwise, you could take your Mac to an Apple Store and ask for a helping hand.