According to Google Documentation (https://cloud.google.com/compute/docs/access/service-accounts): "After an instance has been created with a set of access scopes, you cannot change the scopes for that instance". Therefore, you need to change the Access scopes before launching the instance.
I am afraid that you won't be able to modify it using the Bitnami Launchpad for Google but you can do it creating an Instance from the Google Cloud Launcher. When you launch an instance from the Google Cloud Launcher you use the "Default Service Account". Therefore, you need to edit the Scopes for that account. Please note that you have to be very careful since editing the scopes for the Default account will allow every instance edit the GCR.
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.