In functions.php change
// Index and Userlist should always be displayed
$links[] = '<li id="navindex"><a href="index.php">'.$lang_common['Index'].'</a>';
$links[] = '<li id="navuserlist"><a href="userlist.php">'.$lang_common['User list'].'</a>';
to
// Index and Userlist should always be displayed
$links[] = '<li id="navindex"><a href="index.php">'.$lang_common['Index'].'</a>';
$links[] = '<li id="navforum"><a href="forum.php">'.$lang_common['Forum'].'</a>';
$links[] = '<li id="navuserlist"><a href="userlist.php">'.$lang_common['User list'].'</a>';
Then go to lang/English/common.php and find
// Stuff for the navigator (top of every page)
'Index' => 'Index',
and replace with
// Stuff for the navigator (top of every page)
'Index' => 'Index',
'Forum' => 'Forum',
That should do as required... but be warned that doing this will make some things not work right.
You will need to change all the following files: delete.php, edit.php, moderate.php, post.php, viewforum.php, viewtopic.php,
<a href="index.php"><?php echo $lang_common['Index'] ?>
to this
<a href="forum.php"><?php echo $lang_common['Forum'] ?>
And also change the following file: header.php
if (in_array(basename($_SERVER['PHP_SELF']), array('index.php', 'search.php')))
to this
if (in_array(basename($_SERVER['PHP_SELF']), array('forum.php', 'search.php')))
Any trouble let me know.