The Lumia 620 is a 512MB device and probably won't run great on Windows 10 mobile.
I would try running the 'Windows Device Recovery Tool' from a laptop/pc to check you have the latest firmware and the latest WP8.1 software.
Just be advised that the app will wipe your phone, so do a backup if you want to keep your settings.
Once at Denim you will then need the Windows Insider App and can only use the Release Ring with 512 MB RAM devices.
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