You'll have to make some changes to the code if you want to do exactly that. Maybe you can move all the blocks in the top position to bottom position, so you won't have anything in the top position and the articles will appear at the top. If you still would like to make the code change, this should do it:
Themes/default/Portal.template.php
echo '
<td id="sp_center">';
if (!empty($context['SPortal']['blocks'][2]))
{
foreach ($context['SPortal']['blocks'][2] as $block)
template_block($block);
if (empty($context['SPortal']['on_portal']))
echo '
<br class="sp_side_clear" />';
}
}
function template_portal_below()
{
global $context, $modSettings;
if (!empty($context['SPortal']['blocks'][3]))
{
if (empty($context['SPortal']['on_portal']) || !empty($context['SPortal']['blocks'][2]) || !empty($modSettings['articleactive']))
echo '
<br class="sp_side_clear" />';
foreach ($context['SPortal']['blocks'][3] as $block)
template_block($block);
}
echo '
</td>';
echo '
<td id="sp_center">';
}
function template_portal_below()
{
global $context, $modSettings;
if (empty($context['SPortal']['on_portal']) || !empty($modSettings['articleactive']))
echo '
<br class="sp_side_clear" />';
if (!empty($context['SPortal']['blocks'][2]))
{
foreach ($context['SPortal']['blocks'][2] as $block)
template_block($block);
}
if (!empty($context['SPortal']['blocks'][3]))
{
foreach ($context['SPortal']['blocks'][3] as $block)
template_block($block);
}
echo '
</td>';