How do I increase my storage

Hi

How or can I increase my stage on a MacBook Pro (Retina, 15-inch, Mid 2015) from 1TB to 2TB?

This info from the apple website gives the following info on storage.

Storage1

• 2.2GHz - 256GB
• 256GB PCIe-based flash storage
• Configurable to 512GB or 1TB flash storage.
• 2.5GHz - 512GB
• 512GB PCIe-based flash storage
• Configurable to 1TB flash storage.

Does this mean there are two PCI slots?

The last person I communicated with on this though I was referring to RAM / DDR memory I'm talking about the HD/Storage.

Thanks