You can construct the viewer with a custom skybox pointing to file paths of your choice.
var viewer = new Cesium.Viewer(‘cesiumContainer’, {
skyBox : new Cesium.SkyBox({
sources : {
positiveX : ‘skybox_px.jpg’,
negativeX : ‘skybox_mx.jpg’,
positiveY : ‘skybox_py.jpg’,
negativeY : ‘skybox_my.jpg’,
positiveZ : ‘skybox_pz.jpg’,
negativeZ : ‘skybox_mz.jpg’
}
})
});
``