If you go into your application folder and highlight garageband, you can press the space bar for a quick look and it will show which version it is.
If you have an older mac like i do (2010) you might not be able to update your garageband and it will not work properly and have major issues that are hard to overcome without knowing how to use garageband practically blindfolded. For example, my background is pitch black where the tracks are blending and almost translucent and many buttons are missing/invisible yet still clickable.
I believe 2016 or 2017 is the last updatable year for MacBooks.. but double check and if possible update through the app store or the apple website which can direct you to the garageband app store link. Theres also a list of compatible versions and info from trusted third party sources you can find from Apple as well.
Good luck!