Download Older Versions

Im a bit confused on how/where to download older versions of photoshop. On the screen for the instructions on how to install photoshop, theres a little tab at the bottom telling me that my mac needs to be updated, or follow these steps to learn how to install older versions. I dont know what steps it means. There is a link right next to it but it only takes you to system requirements, and the instructions shown on the screen I thought were for the newer version. Please help!