I investigated the long convoluted chain of button use, and there doesn't appear to be a way to just substitute text for the image.
You would need to edit /includes/templates/your_template/templates/tpl_product_next_previous.php in this area:
PHP Code:
<p class="navNextPrevCounter"><?php echo (PREV_NEXT_PRODUCT); ?><?php echo ($position+1 . "/" . $counter); ?></p>
<div class="navNextPrevList"><a href="<?php echo zen_href_link(zen_get_info_page($previous), "cPath=$cPath&products_id=$previous"); ?>"><?php echo $previous_image . $previous_button; ?></a></div>
<div class="navNextPrevList"><a href="<?php echo zen_href_link(FILENAME_DEFAULT, "cPath=$cPath"); ?>"><?php echo zen_image_button(BUTTON_IMAGE_RETURN_TO_PROD_LIST, BUTTON_RETURN_TO_PROD_LIST_ALT); ?></a></div>
<div class="navNextPrevList"><a href="<?php echo zen_href_link(zen_get_info_page($next_item), "cPath=$cPath&products_id=$next_item"); ?>"><?php echo $next_item_button . $next_item_image; ?></a></div>
Replace the
<?php echo $previous_image . $previous_button; ?>
and similar parts with the text you want. The quick and dirty way would be to put the text directly in the code; if you want to use the official method, or use multiple languages, you need to echo constants which you then define in a language file. You can use BUTTON_RETURN_TO_PROD_LIST_ALT, BUTTON_PREVIOUS_ALT and BUTTON_NEXT_ALT for the text (/includes/languages/english/button_names.php).
PHP Code:
<p class="navNextPrevCounter"><?php echo (PREV_NEXT_PRODUCT); ?><?php echo ($position+1 . "/" . $counter); ?></p>
<div class="navNextPrevList"><a href="<?php echo zen_href_link(zen_get_info_page($previous), "cPath=$cPath&products_id=$previous"); ?>"><?php echo BUTTON_PREVIOUS_ALT; ?></a></div>
<div class="navNextPrevList"><a href="<?php echo zen_href_link(FILENAME_DEFAULT, "cPath=$cPath"); ?>"><?php echo BUTTON_RETURN_TO_PROD_LIST_ALT; ?></a></div>
<div class="navNextPrevList"><a href="<?php echo zen_href_link(zen_get_info_page($next_item), "cPath=$cPath&products_id=$next_item"); ?>"><?php echo BUTTON_NEXT_ALT; ?></a></div>