either (plain js)
this.checked = false;
or (jQuery)
$(this).prop('checked', false);
// Note that the pre-jQuery 1.6 idiom was
// $(this).attr('checked', false);
See jQuery prop() help page for an explanation on the difference between attr() and prop() and why prop() is now preferable.
prop() was introduced with jQuery 1.6 in May 2011.