I think I have at least 10 old Garmin watches from my family. All of them often experienced battery problems after updates. But a simple solution has always worked perfectly for us. Always turn off and restart the watch after the update is complete. Also, check all apps, data fields,... for updates. Apps that are no longer maintained often cause the battery drain. If in doubt, delete old apps and look for a new one.
May not be suitable for all battery problems, but has always worked for us.