Simply drag the model file browser into the URL field on the model component you wish to update, you can also right click copy and paste the assets URL into that same box. Be sure to change the name and use the Save button to save a new version. If you wish to overwrite the old asset, simply do not update the name and click Save. Using this method will allow you to update the Entity tree with your new model component without adding or changing other aspects of your scene. Updating a model component will however also update its material and you will lose any changes made in the studio unless you save the material changes to a shader in your library and re-apply to the newly updated model component, save after updating.