You can switch your Dropbox Business account subscription from monthly to yearly at any time:
- Sign in to dropbox.com with your admin credentials.
- Click Admin Console.
- Click Billing.
- Click Manage subscriptions.
- Click Switch billing period.
- Select the billing period you'd like.
- Click Switch plan.
You'll begin your new annual subscription immediately, and any credit from your past subscription will be automatically applied to your purchase.