Please note that only the account Owner has access to Billing and Invoices and can modify the Subscription.
As the account Owner, you can change your subscription from a Yearly to a Monthly plan from your Account Portal.
From your account, go to Subscriptions. Choose the subscription that you wish to edit.
Click on Edit Subscription under the Details tab.
Once there, toggle from Yearly to Monthly, and click Update.
Any excess payments will be added as credits under your account and can be used on your future bills.
The same process can be followed when changing from Monthly to Yearly and additional payments will be invoiced or charged by card.