Theme supports only one menu. I need to add more – but how? [closed]

You need to add a new menu here:

function register_nisarg_menus() {
        // This theme uses wp_nav_menu() in one location.
        register_nav_menus( array(
            'primary' => esc_html__( 'Top Primary Menu', 'nisarg' ),
            'my_new_menu' => esc_html__( 'My New Menu', 'nisarg' ) //this is the new line
        ) );
    }

and you use it like this in your HTML code to output the menu, using the ID:

<?php
    wp_nav_menu(array(
       'theme_location' => 'my_new_menu'
    ));
?>

you can read more about the menus here.