it's very simple only you need to go to your WHMCS template file and edit your header.tpl and replace your title code to :
<title>
{if $groupname}{$groupname}
{elseif $templatefile eq "configureproduct"}{$LANG.orderconfigure}
{elseif $templatefile eq "viewcart"}{$LANG.cartreviewcheckout}
{elseif $templatefile eq 'checkout'}{$LANG.orderForm.checkout}
{elseif $templatefile eq 'domainregister'}{$LANG.registerdomain}
{elseif $templatefile eq 'configuredomains'}{$LANG.cartdomainsconfig}
{elseif $templatefile eq 'configureproductdomain'}{$LANG.domaincheckerchoosedomain}
{else}{$pagetitle}{/if}
{if $kbarticle.title} - {$kbarticle.title}{/if} - {$companyname}
</title>
