I also had a terrible time trying to update to the new v. 6. First, SDC prompted me to update itself, which I did. After the auto restart of SDC, I selected WindowBlinds 6 and read the "History" information, which also stated what version of SDC was necessary before updating to WB 6. I checked the just updated version and it was the correct one.
I then, according to the destructions, tried to uninstall v. 5.5 but it failed because it said some file (sorry, can't remember which one, but I
think it was a .xml file. Then I tried to download/install v. 6 but got the notice that the previous version had to be uninstalled. Okay... rebooted and tried to uninstall v. 5 again but with the same result. So, I went to the WB installation folder and removed all files except the 2 "validation" files and my downloaded skins. Returned to SDC and got a notice that it appeared that v. 5 was uninstalled and did I want SDC to indicate that it was no longer installed. I of course, choose, "Yes".
Finally, I tried to download/install v. 6 again and this time it worked. No problems thereafter with the download, installation or running it..

This new version is marvelous.... incredible.... fantastic... and I like it too.