There are two ways to update your customer's personal details through Billsby. If you're using the Account management self service modal, your customer will be able to update their information themselves, by logging in and choosing which information to update.
You'll need to make sure these options are toggled on in the self service configuration (Settings > Configuration > Checkout and account management > Self service).
If you're not using the self-service modal, or would just like the make the changes yourself, you can easily do this in the Billsby app. Once you've logged into your account, navigate to the Customers page. Use the search bar to find the customer you're looking for and go through to their profile. Once you're on their profile, you'll see the Edit customer button in the top right hand menu.
When you click on this, the side model will open where you'll be able to input the new details you have for the customer.
Make sure you click on Save changes, to complete the update.