I Want To Stay In The Same Window Instead Of Popping Up A New 1

now I have this calendar program and when you click on a calendar date with an event it pops up in a new window, now I want it to pop up in the same window

the below code makes the selected event popup

function popupEvent(day, month, year, w, h) {
var winl = (screen.width - w) / 2;
var wint = (screen.height - h) / 2;
win = window.open("popup.php?day=" + day + "&month=" + month + "&year=" + year + "");
if (parseInt(navigator.appVersion) >= 4) {
win.window.focus();
}
}