[CSO-286] Add payment method management instructions to billing FAQ

nexus/billing.mdx

@@ -203,6 +203,21 @@ Go to **Billing** \> **"Manage Billing"** in the Stripe portal. Your access cont
     - Slack #ask-civic channel: [join here](https://join.slack.com/t/civic-developers/shared_invite/zt-37tv9fyo7-aDT43mUjOFQwdQFmfZLTRw)
     - Email: [support@civic.com](mailto:support@civic.com)
   </Accordion>
+
+  <Accordion title="How do I change or remove my payment method?">
+    To add, change, or remove your Nexus payment method:
+
+    1. Click on your profile in the bottom left corner
+    2. Go to **Settings** > **Billing**
+    3. Click **Manage Billing**
+    4. In the Stripe billing portal, you'll be able to:
+       - Add additional payment methods
+       - Select a default payment method
+       - Delete payment methods you no longer require
+
+    <Note>You must have at least one valid payment method on file if you're on a paid plan.</Note>
+  </Accordion>
+
   <Accordion title="My payment method failed. What should I do?">
     If your Nexus payment fails, the system will automatically retry the charge. Your account may be temporarily paused but will be reactivated as soon as a payment succeeds.