remove or customise icon found in ‘previous_post_link()’ and ‘next_post_link()’

Use the following code to remove the default backward and forward icons.

Original:

<?php previous_post_link() ?>
<?php next_post_link(); ?>

Change to:

<?php previous_post_link('%link'); ?>
<?php next_post_link('%link'); ?>

or

<?php next_post_link('%link', 'Previous'); ?>