On your account settings page, click Manage billing to open the Stripe portal, then click Update plan. Stripe handles proration automatically — if you switch from monthly to annual mid-cycle, you get credited for the unused portion of the current month against the annual price.
Switching from annual to monthly applies at the end of your current annual period; we don't refund partial annual periods.
The pricing displayed in the portal is always the live, current price — never an out-of-date copy. If you have a question about an invoice, reply to it directly.