how to update my address

Showing where exactly?

The billing address can be edited for each payment method on your account.

click here ➜ If you need to change or update your Apple ID payment method - Apple Support

Or you can edit your general shipping address on a Mac in System Settings ➜ Apple ID ➜ Payment & Shipping ➜ Shipping Address.

Where on your iPhone are you seeing this billing address exactly?

If its linked to a particular payment method on your Apple ID, then follow the steps in the link provided above to find the payment method and update it.

Otherwise you need to provide more context and detail about what billing address you are trying to update.