how to upgrade from 10.13.5 to macos 10.15.6

I want to install xcode to my macOS High Sierra bu it requires 10.15.5 while my system is 10.13.5 so I wanted to upgrade it to the version but it difficult to in updating section on my appstore, this make me want to perform a Recovery and I follow the recovery process now I want to know how do I get the required version