In order to upgrade your plan for sending more no of emails using Netcore services, you can follow below steps,
1. Login to your Netcore Email API account URL:- https://email.netcorecloud.com/index.php/
2. Go to the billing section as shown in below image
3. Click on "Change Plan".
4. Choose the plan basis the approximate email volumes you plan to send monthly and click upgrade.
5. Your plan will be upgraded immediately.