How to assign my home button function to a custom launcher?

I installed Apex Launcher on my S3. It takes some configuring before it is useful, so I didn't assign it to the default action of my home key. I then did accidently set the default action to use the standard launcher. I can't find any way to now set the Apex shell/launcher/what ever it is to be the default when you press home. Any ideas how to change my home button function?