Here is the code for previous post
<?php
$prev_post = get_previous_post();
if (!empty( $prev_post )): ?>
<a href="<?php echo $prev_post->guid ?>" class="previous">
<i class="fa fa-angle-double-left" aria-hidden="true"></i>Previous
</a>
<?php endif ?>
Here is the code for next post
<?php
$next_post = get_next_post();
if (!empty( $next_post )): ?>
<a href="<?php echo $next_post->guid ?>" class="next">Next
<i class="fa fa-angle-double-right" aria-hidden="true"></i>
</a>
<?php endif ?>