You can modify your subscription at any time. Simply navigate to Settings > Billing > Subscription.
Note: Only admins can modify the subscription.
If you choose to upgrade your subscription or subscribe to add-ons, you'll pay a pro-rated amount for the rest of the month.
If you choose to downgrade or remove the add-ons, you'll be credited on the following month's bill. The exception is the Email Finder & Verifier add-on and Credits add-on. If you remove those, the credits will remain available till the end of the current billing period.