Hi,
Downgrading a Windows 8 PC to Windows 7 is not recommended.
If you boot into the bios (f10) and look under the Security Topics -- Secure Boot Configuration you should discover a screen that will allow you to set legacy mode to enable and Secure boot to disable. Also, Disable fast boot.HP Secure Boot Windows 8 Topics
The issue you will find is locating Windows 7 drivers.
I prefer to use Start8 verses Classic Shell but each has its own goods and bads.