Login Admin dashboard.

Go Appearance >> editor on the right side you find “Templates” below that you find list files search functions.php there

add_filter( 'the_content', 'next_prev_posts' );
function next_prev_posts($content) {
$pages = '';
if (is_single()) { // only shows on single post, excluding is_page, is_attachment()
$prev_post = get_previous_post();
if (!empty( $prev_post )) { // previous post available
$pages .= "Prev Post: $prev_post->post_title";
}
$next_post = get_next_post();
if (!empty( $next_post )) { // next item available
$pages .= " | Next Post: $next_post->post_title";
}
}
return $content . $pages; // put the navigation at the end of the post content
}

Add this code and update the page.

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Hope this will Help.

Thanks.