Click "More" in the lower right-hand corner of the page on iOS, select "Membership Details" followed by "Change My Plan." From there, click "Item Upgrade" to upgrade your subscription plan and receive more item(s) in your boxes. Alternately, you can click the "Add 1 More Item" upgrade link located at the bottom of the At-Home section of your virtual Closet.
Plan upgrades take effect immediately, and plan upgrade charges are billed immediately. Unless you downgrade your plan within the first 24 hours after initially activating your subscription, your plan downgrade will take effect at the start of your next billing cycle.