You'll need a Windows 7 license sticker. I boot to and install Windows 7 with a Dell DVD [see update below], activate it using the sticker, and then use the Windows 10 DVD that you can download with the Media Creation Tool from Microsoft to upgrade (within Windows).
Correction (July 2018): The Dell, Windows 7 DVD won't work easily anymore for the Windows 10 upgrade unless the machine is NOT a Dell (or is older than Windows 7). If you have a Dell from the Windows 7 era, use a non-Dell, OEM, Windows 7 DVD instead. Once installed, change the product key to the one from your license sticker (if you haven't entered it already). Confirm that Windows 7 has activated (using that key). Then disconnect from the Internet and proceed with using your Windows 10 DVD as mentioned above.