When trying to renew Core subscription from within the application, no option allows you to reduce the number of subscriptions. It seems you can only upgrade or renew existing number of licenses/subscriptions.
In Core, you cannot downgrade your existing subscriptions from within the application. However, you can purchase a new package with lesser number of subscriptions on the last day of your current subscription. In other words, you have to buy a new subscription plan instead of renewing the existing one. You can also call Technical Support and have them manually change your subscriptions. In that case, BQE Software will issue an invoice manually for that payment. Check the Core Help for details.