In \templates\default\header.tpl find this starting on or around Line 44:

HTML
	<table class="popup_menu">
		<tr>
			<td class="popup_menu_content"><a href="{ubb url="ubb=activetopics&range=7&type=t"}">{$lang.ACTIVE_TOPICS}</a></td>
		</tr>
		<tr>
			<td class="popup_menu_content"><a href="{ubb url="ubb=activetopics&range=7&type=p"}">{$lang.ACTIVE_POSTS}</a></td>
		</tr>
		<tr>
			<td class="popup_menu_content"><a href="{ubb url="ubb=activetopics&range=7&type=u"}">{$lang.ACTIVE_NO_ANSWERS}</a></td>
		</tr>
	</table>

Where you see the 3 instances of &range=7 you can change that value to:

1 for Past 24 Hours
2 for Past 48 Hours
7 for Past 7 Days (default)

GangsterBB.NET (Ver. 7.6.1.1)
PHP Version 5.6.40 / MySQL 5.7.23-23 (was 5.6.41-84.1) / Apache 2.4.54
2007 Content Rulez Contest - Hon Mention
UBB.classic 6.7.2 - RIP