You can manage your Luma Plus subscription by:
Open lu.ma/home/calendars
Choose the Calendar you have Plus for
Go to Settings → Luma Plus
There you will be able to update your payment method, change your plan, or turn off auto-renewal.
Note: We are not able to process refunds for Luma Plus but you can cancel at any time. If you cancel, your plan will be active until the current period is over.