function changeWindow(obj) {
    if(obj.value == 1) {
        window.open("url1", "windowname");
    } else if(obj.value == 2) {
        window.open("url2", "windowname");
    } else if(obj.value == 3) {
        window.open("url3", "windowname");
    } else if(obj.value == 4) {
        window.open("url4", "windowname");
    } else if(obj.value == 5) {
        window.open("url5", "windowname");
    } else {
        window.open("url", "windowName")
    }
}

<select onchange="changeWindow(this);">
<option value=1></option>
<option value=2></option>
<option value=3></option>
<option value=4></option>
<option value=5></option>
<option value=6></option>
</select>