There's a few different ways to do it, but assuming you are using our default collection of view modes, this is probably the easiest way.
var viewer = new Cesium.Viewer('cesiumContainer');
var baseLayerPickerViewModel = viewer.baseLayerPicker.viewModel;
baseLayerPickerViewModel.selectedImagery = baseLayerPickerViewModel.imageryProviderViewModels[1];
If you are defining your own collection of view models, pass "selectedImageryProviderViewModel" as an option to the Viewer constructor with the view model you want along with imageryProviderViewModels, which is the full set.