You change the quantity of boxes in your subscription through your customer account. Here's how to:
1. Log in to your customer account.
2. Go to "My subscriptions" in the left side menu line.
3. Press "Edit" next to your current subscription.
4. Press "Edit" under the headline "Change quantity".
5. Choose your new number of boxes in the subscription and press "Update quantity".
Please note that the deadline for making changes to your upcoming delivery is Friday at 23.59.