You can only upgrade from Linux Mint 19.3 64-bit version to Linux Mint 20. The following upgrade process will not work for Linux Mint 19.3 32-bit version. If you are running Linux Mint 19.3 “Tara”, here are the steps you need to perform to upgrade to the latest Linux Mint 20.

Source: LXer – How to Upgrade to Linux Mint 20 from Linux Mint 19.3