The 620 is past the 3 year release threshold Dell seemed to have used for the release of Windows 10.  Looking at the support page for the 620 it appears they will not be releasing any updates for Windows 10 as they never did for Windows 8.1.  This is normally due to the chip set companies not releasing any drivers to Dell for Windows 10.  At this point if you want Windows 10 you will have to upgrade your computer.