Your 2011 MBP cannot upgrade to any OS higher than macOS High Sierra as it does not meet the hardware specifications for
macOS Mojave or macOS Catalina, namely the graphics card is not 'Metal' capable.
You can check here,
How to upgrade to macOS Catalina – Apple Support
How to upgrade to macOS Mojave - Apple Support
How to upgrade to macOS High Sierra - Apple Support
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.