in product.tpl, find
Code:
{if $config.Appearance.show_in_stock eq "Y" and $config.General.unlimited_products ne "Y" and $product.distribution eq ""}
<TR><TD width="30%">{$lng.lbl_quantity}</TD><TD nowrap><SPAN id="product_avail_txt">{if $product.avail gt 0}{$product.avail}</SPAN>{else}{$lng.txt_no}{/if} {$lng.txt_items_available}</TD></TR>
{/if}
replace with
Code:
{if $config.Appearance.show_in_stock eq "Y" and $config.General.unlimited_products ne "Y" and $product.distribution eq ""}
<TR><TD width="100%">
{if $product.avail gt 0}
IN STOCK
{else}
BACKORDERED
{/if}
</TD></TR>
{/if}