I'm pretty confident there will be no more updates for the 820. The last was over 3 years ago. The 820 has been superseded by the 830 and now the 840. I had been using a small external cylindrical power bank to power my 820 as the battery life had gotten pretty poor. It worked well but it's not something that should be done in the wet. I just got an 840 and the battery life is redonkulous by comparison.
The latest version of firmware is 12.70 which was released over three years ago. It'll show at the bottom of the screen when booting, or you can access the version in Settings, System, About. It will also show in Garmin Connect and Garmin Express. There may be more recent updates to the the gps firmware, time zones, IQ apps, or maps that apply more broadly across other devices, but the 820 operating firmware hasn't changed.