Update WordPress attachment meta

update_meta.php

$args = [
    'post_type' => 'attachment',
    'post_mime_type' => 'image',
    'numberposts' => -1,
    'post_status' => null,
    'post_parent' => null,
];

$attachments = get_posts($args);

foreach ($attachments as $post) {

    // empty anything in content (when we update it at the end!)
    $post_update = [
        'ID' => $post->ID,
        'post_content' => '',
        'post_title' => ''
    ];

    // if we have content, move it to the caption
    if ($post->post_content) {
        $post_update['post_excerpt'] = $post->post_content;
    }

    // if we have any exerpt, copy it to the alt text field
    if ($post->post_excerpt) {
        // update the post's meta-data
        update_post_meta($post->ID, '_wp_attachment_image_alt', $post->post_excerpt);
    }

    // update the post
    $update = wp_update_post($post_update, true);
}