You manage your plan and payment method from Billing (open it from your account menu). Here's how to make changes.
Change your plan
Under Change plan, review the plans available to your account. Choose Monthly or Annual. Annual billing gives you 2 months free.
Choose Preview change to see exactly what happens before you commit:
Upgrading: you're charged the prorated difference today, and your new plan starts right away.
Downgrading: your new price starts on your next renewal date. You keep your current plan until then, and nothing is charged today.
Then choose Confirm change.
Update your card
Under Payment method, choose Add card and enter your details. A new card becomes your default automatically. If you have more than one, choose Make default or Remove on any card. You can pay by card or bank account (ACH).
See your invoices
Under Invoices, choose View for the hosted receipt or PDF to download one.
Where your receipts go
By default, invoices and receipts go to your login email. To send them somewhere else — your bookkeeper, say — find Billing email on the Billing page, enter the address under Receipt email, and choose Save. Leave it blank to go back to your login email.