You'll most likely need to reformat the Win8 partition forst as I don't think mS likes tou to install an older windows over a newer one. download the Parted Magic iso and born it to a CD. It runs from the CD and will let you reformat the partition (NTFS) so the Win7 installer sees an empty disk.
Out of interest, why do you want Win7, if it's just for the start button, you can find Win8 Start buttons easily enough look here:
How to bring back the Start menu and button to Windows 8 | ExtremeTech
Don't forget, if you bought the computer with Win8 preinstalled, you'll need to go into the UEFI (BIOS) and set Legacy boot on, otherwise Win7 may not install and Parted Magic won't run (even from a CD). While you're there, make sure boot from USB and boot from CD are above hard disk in boot order.