There are two ways to do this:
1. Customers can either login to the customer portal and add/update their payment information, provided you've enabled the customer portal for your Chargebee site.
2. Using the "Request payment method" option, you can email customers a secure link to our PCI compliant hosted pages where they can update their payment details. Once they've successfully completed it, the details will be updated in their account.
The "Request payment method" option can be found in the Customer’s page (Subscriptions > Customers > Select the Customer > Request payment method (on the right))