How to download old version of adobe photoshop?

To access old versions, check the box in your CC Desktop Preferences (see screenshot).

Adobe assumes you have the latest OS & software, no more than 2 versions back.  Thus only the last 2 major release versions are available from CC Desktop App. 

Please ensure your computer meets or exceeds the minimum system requirements to run Creative Cloud + OS + all other apps you use. Check each one.
- https://helpx.adobe.com/creative-cloud/system-requirements.html

Also note Photoshop's new GPU requirements. See links below.
- https://helpx.adobe.com/photoshop/system-requirements.html
- https://helpx.adobe.com/photoshop/kb/photoshop-cc-gpu-card-faq.html

Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-the prodesign links go directly to Adobe's file server, so this process will
--only work until Adobe completely removes the old version(s) from the server
.
IF/When you do have a download, be sure to copy it to AT LEAST one external device
-since Adobe does sometimes remove old versions from the server
-a usb flash drive or external hard drive, or write to a DVD
-plus a text file containing your serial number if you have CS6/CS5