how to overwrite next_post_link

You can’t easily change only the URL of that link, because there is no filter that will allow you to do that, but…

next_post_link uses get_next_post_link. There are no filters in any of these functions, but… get_next_post_link uses get_adjacent_post_link and inside of that function you can see {$adjacent}_post_link hook.

So now we can check docs for that hook. And then write some code:

function my_next_post_link($output, $format, $link, $post, $adjacent) {
    return '<a href="https://wordpress.stackexchange.com/questions/325546/<MY LINK">MY LINK LABEL</a>';
}
add_filter( 'next_post_link', 'my_next_post_link', 10, 5 );

PS. Most likely you also have to add some conditions in there, so you don’t change every link on your site and only the one you want to.