Actually for my son, who needs Photoshop CS6, which only runs on Yosemite, not El Capitan, so need a good service center to backup the current OS (El Capitan) completely to external disk, and then install Yosemite. The Mac is a only about 2 months old, a "Retina".

PS: I don't think his machine was upgraded from Yosemite, so I doubt that he'd have the simple option to "rollback" a version of Yosemite which was already installed on his system (ie I think the first OS X installed on this machine was El Capitan)

If it's a reasonably new Mac, it's also unlikely that you'll be able to install an earlier operating system. Sometime back, Apple decided to make all computers compatible only with the system they had initially installed, and upgrades. Nothing previous was possible.

Good luck. Should be able to get a more recent version of Photoshop that will work with El Capitan.

https://helpx.adobe.com/photoshop/kb/photoshop-and-el-capitan-mac-os-10-11.html

Not so long ago I upgraded from Yosemite to El Capitan and have no problem with Adobe Photoshop CS6....