Boxes cannot be skipped. The kits ship in a fixed order, and the only choice built into the plan was which box arrived first, picked when the subscription was set up.
STEM Box subscriptions are closed to new sign-ups, so taking out a short subscription just to get one particular box is no longer possible either. Existing subscriptions have since been wound down: rather than continuing month by month, the boxes still owed were consolidated and sent as one final delivery.
If there is one kit you particularly want, the simplest route is to buy it on its own. The kits that came in the boxes are sold individually in our shop.
To stop a subscription that was never cancelled from renewing, see How do I cancel my subscription?.