2
Easy! Open your theme.html file in your text editor. Look for this line near the beginning of the body text:
<td width="10%" height="25" background="<{$xoops_imageurl}>navbar2.gif" style="border: 1px none rgb(0,0,0)"><small><i>little slogan</i></small></td>
and change the text "little slogan" to whatever you want it to say.
Peace