Try upgrading to macOS 10.11 El Capitan first. You can find the installer within this Apple article which also includes instructions for creating a bootable macOS USB installer:
https://support.apple.com/en-us/HT201372
Make sure to read the article carefully as there are several steps to perform to extract the downloaded installer to the Applications folder. I recommend using a bootable macOS USB installer so you have more options if something goes wrong.
Also you may be better off performing a clean install by first erasing the whole physical drive before installing the new OS since most of your third party apps will not be compatible with macOS 10.15 or 11.2 since 32 bit apps are no longer supported with those versions of macOS.
You want to make sure you have the very latest version of the Big Sur installer which should be version 11.2.1 plus you want to make sure you have at least 46GB of free storage space in order to install Big Sur.
Make sure to have a good backup before you attempt to upgrade macOS.