Solved Upgrade desktop with Windows 7 to Windows 10 ?

I work as a janitor at a church and they have asked me if I could upgrade one of their desktop computers from Windows 7 to Windows 10. I told them I would get back to them about that. I don't have the computer yet so I can't offer any specs on it, but I do have a little bit of info on it - see paragraph below.
My question is this: the computer came with Windows 7 and is still currently running Windows 7. I would like to do a clean install of Windows 10 on the machine and I do have a Windows 10 ISO fill on a USB flash drive (1803) version that I could use to do the upgrade. The computer does have a Windows 7 product key on the case, so if I was to do this I could enter that product key at the time of installation but was wondering if that product key would be accepted for activation of the Windows 10 upgrade? I want to be sure I am not committing any piracy stuff here and want to be perfectly legal. I am not interested in doing this unless it is perfectly legal. Your help would be appreciated. If this is legal I'd like to help them out and save a little money. Otherwise they will need to purchase a Windows 10 license and I can still do the upgrade for them with the new license.

 

My USB thumb drive has Windows 10 Home version. I believe the desktop is Windows 7 Home Premium. Yes, the church has other computers they can use if something goes wrong. I also have access to a computer with Windows 10 Pro so I could always make a USB thumb drive with that ISO on it as well, so either way I would have the correct Windows 10 ISO that I could use.
The more important question I'm trying to get to here is doing this upgrade a legal thing to do as long as I have the Microsoft product key for the Windows 7 desktop system that I would be upgrading?

 

Thanks Frank (I hope it's okay to just call you Frank instead of Mr. Flavallee).
I know that during the installation process using the USB drive ISO file, it asks for the user to type in their Microsoft Product Key. Now you can skip that and it will still install the upgrade but within 30 days it will start nagging you to enter the product key. What happens, I understand is that once the Product Key is entered during the installation, that number gets sent back to the Microsoft servers for authentication. If it can't be authenticated you might just stop right there. Anyway, be-what-it-may, that is my understanding but I do have a legitimate Windows 7 Product Key so it should be good to go.

I have to ask "what is the method that you use to upgrade Windows OS's (if you don't mind sharing that) ?

 

Thanks Frank - as always you've been a big help