$5.9/month or $59/year for the FIRST device
$4.0/month or $40/year for the SECOND device
$3.5/month or $35/year for the THIRD device
$3.0/month or $30/year for the FOURTH and further devices
You do not need to do anything, it is already reflected in the mobile App. Just go to 'Details' -> 'Subscription' and you will see a reduced price based on the number of active subscriptions that you have.