I don't know if you can activate Windows 10 as such but if you still have recovery partition then I would suggest to install Dell backup and recovery manager from this link and see if it allows you to factory restore.
Edit:As pointed out by @Moab this software will not work in current scenario then I would recommend downloading the latest .iso of windows 10 from this link and then you can activate normally with your unused windows 7 key as clarified in the question comment.