1. Login to your account and choose subscriptions:
2. Then choose needed subscription:
3. In the subscription details choose 'Upgrade or downgrade':
4. Then choose needed plan:
and needed (upgraded) amount will be placed to the cart:
Note: needed amount is total amount (not added to current amount). E.g. if there were only 2 items and needed additionally 4 items then upgrade should be chosen to 6 items (2 current + 4 new).
5. Process the checkout.
Note: Charging will be made only for the price difference.