The easy way would be to buy an external harddrive and backup your data. Then install Windows 8 while rebuilding the harddrive to a single partition.
The hard way involves changing core system files and deleting files. But you need to be extremely careful because making the wrong change will result in having to start over using the easy way and hoping that your data is still intact.
Either way you should always backup your data before upgrading your OS.
or go into control panel / system & security / system / advanced system setting - advanced - startup and recovery and remove the option to boot to the other OS.
You should then be able to remove the other OS's its not as clean as it could be, you'll then have to expand the partition of the surviving OS to take up the remaining space or create a new partition out of them.