Edit in JSFiddle

$(document).ready(function () {
    //target the ul you desire to process
    var $ul = $('.target-ul');
    //grab the last 6 lis, and pull them off the dom
    var $last6lis = $ul.find('li:gt(5)').remove();
    //make your new ul, to put the dom elements in
    var $newUl = $('<ul></ul>');
    //append the elements back on the DOM - easy :)
    $ul.after($newUl.html($last6lis));
});

<ul class="target-ul">
    <li>Item 1</li>
    <li>Item 2</li>
    <li>Item 3</li>
    <li>Item 4</li>
    <li>Item 5</li>
    <li>Item 6</li>
    <li>Item 7</li>
    <li>Item 8</li>
    <li>Item 9</li>
    <li>Item 10</li>
    <li>Item 11</li>
    <li>Item 12</li>
</ul>

ul {
    margin-top:12px;
    background-color:rgba(128, 128, 64, 0.2);
    width:48%;
    display:block;
    float:left;
    margin-left:1%;
    border-radius:5px;
    padding:0;
    list-style-type: none;
    border: 1px solid #cccccc;
}
ul li {
    text-indent:8px;
    font-family: Helvitica, Calibri, Arial;
    font-size:18px;
}