def url_replace(req, field, value):
    """
    Replace the old GET value of desired field with a new value.
    
    Args:
        req: the http request
        field: the field to replace
        value: the new value
    
    Returns:
        The updated url with the new value
    """
    dict_ = req.GET.copy()
    dict_[field] = value
    return dict_.urlencode()