Power off your Dell. Remove the AC adapter and Remove the battery. Your Windows 7 COA should be present there. Use your phone and take a picture of the COA. Then replace the battery, AC adapter and power the system on. Try to activate Windows 10 TH2 using the Windows 7 OEM Product Key.
I have upgraded my laptop OS from Window 7 to Window 10 when window 10 launched. Now message appear on bottom right corner than please active windows (Go to settings to activate windows). But my window 7 was genuine and it came with my laptop pre installed. Then how come Windows 10 become pirated.  I lost my Windows 7 produce key. sticker also not available. now how can I activate my Windows 10 . Microsoft online help center said that they are unable to active your Windows 10 without product key. My Dell laptop model is Dell Studio 1555. 
Window 7 genuine while upgrade to Window 10 than it becomes pirated