To do a clean install you will need first to uninstall all previous Sitefinity 3.0 versions. Do this from the Add/Remove programs tool in Control Panel. Another way to uninstall your old Sitefinity version is to delete all files except the project ones under Program Files\telerik\Sitefinity 3.0
. This way you should remove the virtual directories too. This should be enough for removing the old version.
Do not forget to back up your project and database.
After uninstalling your old version and installing the new one follow the instructions from the upgrade file we have already sent you.
Let us know if you experience problems while trying to upgrade your project.