Hi Ian, you can reinstall Windows 10 at any time, you will not need a product key, activation will be automatic and it will not cost you anything !
You should just clean install Windows 10 rather than upgrading from Windows 7. This will have 2 benefits, if you have to reset, it will reset to Windows 10, not 7 and your system will be more responsive and reliable . . .
Since you previously had Windows 10 installed and activated on that PC during the installation process skip the steps which ask for a product key and select the option 'I am Reinstalling Windows 10 on this PC', and activation will not be an issue, your PC will have a digital entitlement stored on the Microsoft Activation Servers
Make sure you install the same Edition of Windows 10 (Home, Pro . . . etc.) you previously had installed to avoid Activation issues
Click this link:
https://www.microsoft.com/en-us/software-downlo...
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 10 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool
Then, Boot your PC from the Installation Media you just created (change Boot Order in your BIOS) to begin installing Windows 10
