Your Windows 7 key should work. You can purchase a key online though if you choose. The Dell support page for the E6430 does list Windows 10 drivers so the system is compatible with Windows 10. If updating the system does not work then download the Windows 10 media creation tool here which will create a bootable Windows 10 installation flash drive. And a very good tutorial video for installing Windows 10 is available on YouTube here.
Hi My most recent machine is a Dell Latitude 7400 but I also have a have a Dell Latitude E6430 running Windows 7 Pro which has a current valid product key. Before its warranty expired in 2016 I tried the free Windows 10 upgrade a few times but each time it failed. I decided then to continue with Win 7. Now with support for Windows 7 long gone, I need to upgrade the E6430 to Windows 10. My question: Is an upgrade to Windows 10 still going to work with the Win 7 product key or must I purchase a new key. I understand keys are available for sale online. Thanks Daniel    
Windows product key