what so ever it is, insert you installation CD/DVD or USB, just follow all the normal installation process, when it come to Custom(Advance) installation option, select that, and format the drive where Windows 8 installed, then continue on that partition.
You will have windows 7 over 8..
This is useful if you need step by step help:
http://getintopc.com/tutorials/how-to-install-windows-7-step-by-step-method-for-beginners/