1. Here's how

(showing Lumion 12)

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 in Build Mode.

1.3: Point the camera at the model.

1.4: Hold down the CTRL key and left-click the thumbnail image in the upper right corner of the screen in Build Mode to update the image.

Before:

After: