Hey there-
I just solved a similar issue on a blog on the Margot template. Here's the code I used, with the appropriate class names taken from Rally. Toss this into your header and let me know if you need anything else!
<script>
(function() {
window.addEventListener('load', function() {
var paginationLabels = document.querySelectorAll('.BlogItem-pagination-link-label');
for (var i = 0; i < paginationLabels.length; i++) {
var text = paginationLabels[i].textContent;
text = text.replace("Previous", "Newer Post");
text = text.replace("Next", "Older Post");
paginationLabels[i].textContent = text;
}
});
})();
</script>