In How to uninstall? tells:
if somebody wants to upgrade from 1.3 to 1.4:Version 1.4 (also earlier versions) will uninstall from the Windows Add/Remove Software control panel without problems.
If you want to completely get rid of Spybot-S&D 1.4 (also earlier versions) and the Add/Remove doesn't help, you can delete the installation folder (usually C:\Program Files\Spybot - Search & Destroy\), then download this very small fix and execute it (important! - this will remove all remains from the registry) (this also applies to earlier versions).
If you just want to upgrade to a newer version, please follow the same instructions like above and then install the new version.
1.- Uninstall
2.- Install new version
ok?
But in the spanish version: ¿ Cómo desinstalar ?:
An that means: "If you want a clean install, I recommend you to install above the present version", without a previous uninstalling.Si usted desea una instalación completamente limpia, le recomiendo que instale encima de la versión actual.