Replace a ‘Title’ tag with a Custom Field

You will want to read up on get_post_meta(). Here is a link to the WP Codex reference for that function.

<?php $meta_values = get_post_meta( $post_id, $key, $single ); ?>

If you created that custom field with something like Elliot Condon’s ACF plugin, the documentation for the plug-in is also helpful in meeting your exact need.

Assuming you are inside the loop so something like this should produce what you want:

$key_title = get_post_meta( get_the_ID(), 'yourkeyname', true );
echo $key_title;