Question

is it possible to use jQuery to do that:

if the body has a <div id="wpadminbar">
then change the document.location.href in this input from

<input type="button" onclick="document.location.href='/wp-login.php?action=register';" value="Forum" id="forum">

to

<input type="button" onclick="document.location.href='/forum';" value="Forum" id="forum">

Answer 1

if($('#wpadminbar').length > 0) {
    $('#forum').attr('onClick', "document.location.href='/forum';");
}