Remova o marcador anterior e adicione o marcador no lat lng atualizado

1. use a referência para mover o marcador existente

var marker;
function displayLocation(location) {
  console.log(location.lat)
    var position = new google.maps.LatLng(parseFloat(location.lat), parseFloat(location.lng));
    if (marker && marker.setPosition) {
      // if the marker already exists, move it (set its position)
      marker.setPosition(location);
    } else {
      // create a new marker, keeping a reference
      marker = new google.maps.Marker({
        map: map, 
        position: position,
        title: 'test!'
      });
    }
}

2. remova o marcador existente do mapa e crie um novo

var marker;
function displayLocation(location) {
  console.log(location.lat)
    var position = new google.maps.LatLng(parseFloat(location.lat), parseFloat(location.lng));
    if (marker && marker.setMap) {
      // if the marker already exists, remove it from the map
      marker.setMap(null);
    }
    // create a new marker, keeping a reference
    marker = new google.maps.Marker({
      map: map, 
      position: position,
      title: 'test!'
    });
}