change the comment time output to: X time ago instead of actual date and time

What you need is: https://codex.wordpress.org/Function_Reference/human_time_diff

So this should do exactly what you need:

 <?php printf( _x( '%s ago', '%s = human-readable time difference', 'your-text-domain' ), human_time_diff( get_comment_time( 'U' ), current_time( 'timestamp' ) ) ); ?>