Can I update from MacOS High Sierra 10:12 to Big Sur

A 2012 iMac can only run as high as Catalina.

Click on this link to read how to get different versions of macOS - hhttps://support.apple.com/HT211683

Before upgrading, back up at least your data in case something goes wrong. See this support document: How to back up your Mac - https://support.apple.com/mac-backup

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. To check for those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/

If you can't upgrade far enough to get the version you wish then you can consider using any one of the many free browsers out there that see longer support than Safari.