To manage your Pro or Business account, go to the Insync homepage and click on
Sign in with Google on the upper right hand corner
After logging in you will then be sent to the dashboard. Click on to add or remove Google accounts from your subscription.
To add account slots to your subscription, click on
subscription and click on to add slots. Click on
update subscription afterwards.
To add or change the credit card on file, click on the side and change the details from there.