First back up head.tpl.
Open head.tpl in your skin directory (probably skin1).
Find this line at the top on line 4.
<td class="HeadLogo"><a href="{$http_location}/"><img src="{$ImagesDir}/xlogo.gif" width="244" height="67" alt="" /></a></td>
Change xlogo.gif to whatever you have your logo named. Save the file.
Upload head.tpl to your skin directory and you logo to skinDir/images