Each DE can have slightly different paths, but in general find the file mimeapps.list. It's typically looked for in the traditional manner of /etc/, /usr/share/ and /usr/local/share/ and $HOME to find global, local and personal configurations. In your $HOME it's usually something like this:

    ~/.local/share/applications/mimeapps.list
    ~/.config/mimeapps.list

In here you find the MIME type pointing at a .desktop file (app launcher), it's a matter of changing that. Mine for example related to using Firefox:

    [Default Applications]
    x-scheme-handler/http=firefox.desktop
    x-scheme-handler/https=firefox.desktop

Find the .desktop file you want to launch and just update that bad boy. Most DEs have a GUI tool to manage this for you without having to resort to manual editing...

Thanks.

I use BROWSER environment variable for that.

It seems to even override XDG settings.

    % xdg-settings set default-web-browser firefox.sh.desktop
    xdg-settings: $BROWSER is set and can't be changed with xdg-settings
    
    % echo ${BROWSER}
    firefox

Hope that helps.

EDIT: You also got me nice idea - to add http(s):// and ftp:// support for mine see.sh opener :)