You may have some third party software installed that is interfering (anti-virus apps, cleaning apps, third party security software -- none of which are needed on a Mac and usually causes problems). Try booting into Safe Mode to see if the upgrade will run since this disables most third party drivers and Login items. Is your Mac being managed by a school or business which may restrict your ability to download the installers?
If you already have a macOS installer downloaded (or even partially downloaded), then you should delete it and empty the Trash. macOS can be configured to download updates in the background. This will make sure a former download is not interfering as well as making sure you have the latest version of the macOS installer (very important).
The link @sberman provided includes instructions for using Software Update and the App Store (with a link to the App Store download of Big Sur). Here is another Apple article for containing links to the various macOS 10.11+ installers as well as instructions for creating an optional bootable macOS USB installer:
https://support.apple.com/en-us/HT201372
You also have access to Internet Recovery Mode (Command + Option + R) which should allow your Mac to boot to the online Big Sur installer (in theory since sometimes Macs will only boot to the online installer for the oldest OS still available for your Mac).
Make sure to have a good backup before you attempt to upgrade the OS.
