Upgrades will install like a normal disk. You just need a product key.
First find a nice stable storage device for your files. Back up everything that's needed. (All my files together are about 100GB)
Next you want to format your disk. You can either format it at the setup of Windows 8 or use a third party disk manager like
Gparted.
Optional* If you want to fix some bugs in your hard drive you can load the windows 8 or windows 7 installation disk and perform a low level format which sets all writable sectors and even some bad sectors back to 0 (nothing)
1. Boot into the windows 7 or windows 8 disk.
2. Choose language and click Next.
3. Choose "Repair your computer" which is in the lower left corner.
4. Choose your windows partition and press next
5. It will try and search for errors at boot. Click Cancel if it asks you to restore or reboot.
Choose "View advanced options for system recovery and support" after you close the notification.
6. Click "Command prompt" and type in the command
diskpart and then press enter.
Next type the command
listdisk and press enter. This will give you a list of disks.
type select disk # and press enter. (Replace # with your disk number)
type clean all (WARNING takes about a day for a large drive)
Reboot and continue with the installation
Insert your windows 8 Release preview disk or update disk (you can get it here)
Choose your language and click next
Click "Install now" and accept the license agreement. Product Key: TK8TP-9JN6P-7X7WW-RFFTV-B7QPF
Do a custom install
Choose your drive and click "Advanced"
Delete ALL your partitions and then click New (If you did the optional step then just click New)
Click ok and then click Next. Let the installer finish and then customize windows 8! Also transfer your files back.