So what you need to do is make your life easier and instead of searching everything within every instance of .pp-post-content or .pp-post-content p, let’s wrap the times or the times in question with a span tag. Like you suggested in your comments <span class="tas-event-time"> is sufficient enough.
Now, in your .js file, you want to add the following:
jQuery( document ).ready( function($) {
$( '.tas-event-time' ).html( $( '.tas-event-time' ).html().replace( '12:00 pm','12:00 noon') );
} );
That will, once the document is ready, change the instances of ’12:00 pm’ to ’12:00 noon’.