If you upgrade to the same major version (e.g. IW 2.1 -> IW 2.2), the old version is replaced automatically. If you upgrade version 2 to version 3, IW leaves the old version there.
You remove it exactly the same way, as any other software on Windows, the uninstaller will remove the plugin from Acrobat folder.
On Mac you will need the original DMG file to use the installer for plugin removal. See here for more details: https://pressnostress.com/support/iwacrobat-help/home_uninstall.html
You can also remove it manually, so if the above doesn't help, let me know your operating system and IW version you want to remove, so I can provide a better instruction.
Customer support service by UserEcho