1. Here's how
The thumbnail images for Imported Models are updated automatically when you import and re-import the models.
1.1: To create your own thumbnail image for an Imported Model, place the model in the Scene.
1.2: Select the model and make sure that the thumbnail image of the model is still selected in the Imported Model Library.
1.3: Point the camera at the model.
1.4: Hold down the CTRL key and left-click the thumbnail image in the lower left corner of the screen in Build Mode to update the image.