blogPostNextPrev.liquid

{% assign ordered_posts = page.parent.children | sort:'published_at' %}

{% for post in ordered_posts %}
{% if post.id == page.id %}
{% assign previous_post_number = forloop.index | minus:1 %}
{% assign next_post_number = forloop.index | plus:1 %}
{% endif %}
{% endfor %}

{% for post in ordered_posts %}
{% if forloop.index == previous_post_number %}
{% assign previous_post = post %}
{% elsif forloop.index == next_post_number %}
{% assign next_post = post %}
{% endif %}
{% endfor %}

You want to wrap your buttons in those liquid {% if %} conditionals so you don’t get a ‘previous’ link on your oldest post, or a ‘next’ link on your newest post.