This code was borrowed from WP Beginner's website: https://www.wpbeginner.com/wp-tutorials/display-the-last-updated-date-of-your-posts-in-wordpress/

snippet.php

<?php
// Download this snippet as a plugin and more at: https://wordbits.io/snippet/display-the-last-updated-date-of-posts/
// Created by: Liam Dempsey
?>

<?php

function wpb_last_updated_date( $content ) {
$u_time = get_the_time('U'); 
$u_modified_time = get_the_modified_time('U'); 
if ($u_modified_time >= $u_time + 86400) { 
$updated_date = get_the_modified_time('F jS, Y');
$updated_time = get_the_modified_time('h:i a'); 
$custom_content .= '<p class="last-updated">Last updated on '. $updated_date . ' at '. $updated_time .'</p>';  
} 
 
    $custom_content .= $content;
    return $custom_content;
}
add_filter( 'the_content', 'wpb_last_updated_date' );

?>