In your admin control panel, go to Display Options - Templates

Look for header.tpl then click on the EDIT SELECTED TEMPLATE button.

Look for

Code
<title>{if $inputTitle}{$inputTitle|strip_tags} - {/if}{if $powered}Forums{else}{$config.COMMUNITY_TITLE}{/if}</title>
as SK suggested.

Replace with...

Code
<title>{if $inputTitle}{$inputTitle|strip_tags} - {/if}{if $powered}YOUR STUFF HERE{else}{$config.COMMUNITY_TITLE}{/if}</title>