Well, it seems like Twitter Counter is useful for you and you need more advanced statistics as your account grows.
Please follow these steps to upgrade:
1. Log in with your account and go to "Settings".
2. Select "Billing".
3. Then click the on the Subscription you would like to Upgrade.
Click "Adjust”, and you’ll be redirected to our pricing page.
Choose the plan you want and hit "Upgrade Now".
You will be charged for the difference between the "old and "new" plan right away.