How do I change the card I pay with?

In Stripe's billing portal for a plan, and on the Vault's own page for a Vault that renews.

• For your plan: click your name or picture at the top right (on a narrow screen, the menu button), then My Account, and click Manage Billing on the My profile page, or View full billing page in Stripe on Plan & billing. Both open Stripe's billing portal, where your card, your details and your invoices are.
• For a Vault that renews by itself: click Dashboard in the top bar, then the Vaults tab, click Open on the Vault and, under Change this Vault, click Update my card or see invoices.

A Vault you paid for once keeps no card, so it has nothing to change. The Panoramr mobile app hides Manage Billing and Plan & billing: use a web browser.