To manage your workspace's subscription in Sympli Versions you can click on the Manage Subscription button in the Workspaces Manager top section. Another option is to get straight to your Subscription Settings from your Sympli Versions Billing Page.
In the upper left corner, you'll find a dropdown titled Workspaces. Click on it and choose a workspace you want to manage your billing info or seats. Click Add Seats to add more new seats to your workspace's subscription.
To remove seats, click on Billing Tab on the left side of the screen. You will be taken to the billing page where you can add or remove seats, and update your card and billing info.