Sometimes the upgrade process can take a very long time due to hardware components and their performance and the stuff Windows has to do before it can install Windows 10 properly. For example, a slow computer packed with lots of applications will take a very long time to finish. I've encountered the same issue and the only thing I did was to wait until it's done.
You can go ahead and reboot the PC and the setup will start again. If it's stuck, you should see or notice through the hard disk activity.