# Update the customer during checkout

Attach a customer to a Checkout Session after creation to support mid-checkout authentication.

Some merchants allow customers to begin checkout as a guest and offer the option to log in during the checkout process. With this feature, you can attach a [Customer](https://docs.stripe.com/api/customers.md) to an existing [Checkout Session](https://docs.stripe.com/api/checkout/sessions.md) after creation, enabling their saved payment methods, email, and billing information to automatically populate without losing any information the customer has already entered.