Hi Tallulahfrombc,
Thank you for using Apple Support Communities. I understand that you're currently trying to update GarageBand while on macOS El Capitan. I'd be happy to provide some insight.
The latest version of GarageBand is version 10.3.4. In order to perform the update to the latest version of GarageBand you must also update your Mac as the lowest supported operating system is macOS High Sierra 10.13.6. The most recent version of macOS available is macOS Catalina 10.15.4.
If you are interested in upgrading your macOS you can do so by visiting the Mac App Store. Here is a great resource on performing an update to macOS Catalina: Upgrade to macOS Catalina.
However, before upgrading your macOS it is highly recommended that you make a backup. If you don't already have a backup that's alright. Here is a resource that can help you make a backup: Back up your Mac with Time Machine
Kind regards.