Edit a single address in Commerce
Are you looking for something like this? The second example will ensure an address variable exists, even if it is set to null.

{% set address = craft.commerce.customer.address(addressId) %}

or

{% set address = address is defined ? address : null %}

As for accessing the data, you would just do this:

{{ address ? address.firstName : '' }}

Another way to do it (without needing to define a null address variable) is:

{{ address.firstName ?? '' }}