How can you go back to your old ubuntu os from an upgrade pop os without using usb , cd , dvd and so on ..? [closed]
"Upgrading" to Pop OS is an impossibility - PopOS is a different Operating System than Ubuntu. It may be ubuntu based but it is a unique system.
To 'go back' to plain Ubuntu you need to do a complete reinstall of Ubuntu. There is no alternative as you cannot 'revert' switching OSes.