Hi,
Apparantly you have overseen something during the installation. Right at the beginning you can select between upgrade an existing installation and a fresh install. Default is the fresh install, so that was installed alongside U6.
Removing U6 and U7 is not that difficult, but be sure you backed up any data that you want to keep before removing them.
After you made the backup, you can start your computer from the U7 or U8 install disk. Instead of letting Ubuntu figure out how to install, you can either choose that Ubuntu uses the entire hard disk (assuming you have only one harddisk) or you choose for a custom harddisk layout, where you can first format all existing partitions (except the swap, that one you can leave untouched), thus wiping out U6 and U7.
Hope this helps!