Battery drain after update to 8.10?

In every beta release is the file to roll back to the last official release for that particular firmware.   The beta installation instructions I believe tell you how to do it but it's literally as simple as copying over that one file and then doing the 'update' when the watch detects there's a new file.  It's important to note this will wipe your settings and you'll have to set it up again. 

Just FYI it's a good reason to download the beta's just so you have access to each version of the official firmware if you ever want to go back in time and test something. 

How can I downgrade?

As Nate said you need a thrid party app. 

He suggested Battery Gauge.

my question was to Aagar. I asked him how he calculated the battery consumption that he stated. 
It is also known that a Battery widget itself consumes battery.

It should also be noted that the calculated remaining time in days displayed by the watch is reduced if you change the background brightness to a higher value. Even if you never use the backlight.

And I asked you how to downgrade, if you suggested it.

"7. If you would like to revert to the last public release software, drop the gupdate.gcd file found in the System_Backdate_v*** folder in the \GARMIN directory NOTE: If you revert to an older version of software, all of your settings will be reset to defaults."

Battery consumption raised from 0.3 % per hour to 0.6 % per hour since 8.10. Not cool