<?php
$post_id = 48;
$post = get_post($post_id);
$content = $post->post_content;

$content = str_replace(
    ".about-connect-card .about-card-title { font-family: 'Cardo', serif; font-size: 42px; font-weight: 700; margin: 0 0 40px; color: #1C3434; }",
    ".about-connect-card .about-card-title { margin: 0; font-size: 42px; line-height: 1; }",
    $content
);

wp_update_post(['ID' => $post_id, 'post_content' => $content]);
echo "Fixed margin.\n";