You can change it in homepage.tpl
now it it is:
<h3 class="members members_list_tabs">
<a href="#" onclick="switch_boxes('members_list','members_popular',1);return false;" class="members_popular_tab">{lang:"core","home_popular_members"}</a>
<a href="#" onclick="switch_boxes('members_list','members_new',1);return false;" class="members_new_tab">{lang:"core","home_latest_members"}</a>
</h3>
change it to:
<h3 class="members members_list_tabs">
<a href="#" onclick="switch_boxes('members_list','members_new',1);return false;" class="members_new_tab">{lang:"core","home_latest_members"}</a>
<a href="#" onclick="switch_boxes('members_list','members_popular',1);return false;" class="members_popular_tab">{lang:"core","home_popular_members"}</a>
</h3>