You can download genuine Windows 7 ISOs direct from Microsoft, just google/bing it. Burn the ISO to a disc using a software like IMG-Burn, or create a bootable USB (I don't know how, it will be on the MS site where the ISOs can be downloaded from). Then boot from the ISO and install windows 7. Since the key is matched with the motherboard you should be fine activating windows 7 with the existing keys. If the keys don't work, you'll have to ring Microsoft automated phone activation to reactivate the keys.
After you have installed windows 7 keep updating and restarting until you get to the windows 10 update. It may take a looong time to fully update windows 7 to the stage, depending on how old the ISOs are.