I need to customize wordpress “previous” “next” links

If I’m understanding this correctly, you may just want to use something like:

$prev = get_previous_post();
$next = get_next_post();

Then to access the title you’d do something like

<h1><?php echo $prev->post_title ?></h1>

And if you wanted its image:

$prev_img = get_post_thumbnail_id( $prev->ID );
<img src="https://wordpress.stackexchange.com/questions/248644/<?php echo $prev_img; ?>" alt="your alt here">