var viewer = new Cesium.Viewer('cesiumContainer');

var lon = -120;
var lat = 35;
function getPositions(){
    lon += 0.05;
    return Cesium.Cartesian3.fromDegreesArray([lon, 35, -125, 35]);
}

var redLine =  viewer.entities.add({
    polyline : {
        positions : new Cesium.CallbackProperty(getPositions, false),

        width : 5,
        material : Cesium.Color.RED
    }
});

var handler = new Cesium.ScreenSpaceEventHandler(viewer.scene.canvas);

handler.setInputAction(function() {
    redLine.polyline.positions = Cesium.Cartesian3.fromDegreesArray([-120, 35, -125, 35]);
}, Cesium.ScreenSpaceEventType.LEFT_CLICK);