Only users with the Full Account permission can update subscription details.
- Go to the "Account Settings" screen.
- Click on the "Billing" tab
- Hover over the "Subscription" area and click on "edit section"
You can click on "Actions > Edit Subscription" as well.
An overlay will appear where you can change the details of your subscription. Please note, you can only downgrade to a subscription which supports the number of active profile records currently on your account.