You will find the conditions for making online seat reservations for SWISS flights here.
The following conditions need to be met in order to make an online seat reservation at swiss.com:
You can state your preference (window or aisle) on all other flights.
Miles & More status customers (Senator and HON Circle) can book seats on any SWISS-operated flight until just before departure. Sign in with your Miles & More profile to choose your seat (no charge, except for Economy Light).