How can i completely uninstall old unity demo?

I can’t completely uninstall old unity? I have uninstalled it and deleted all the files and when I download the new version it still not play. How can I fix it, any ideas?

I’m having the same issue… I am using a macbook pro. Unity4 trying to roll back to Unity3.5
I remove Unity + the license. deleted all from the Garbage bin etc… re-started computer re-intalled but still I get an error