How can I get more gb

As stated aboce, there’s no hardware upgrade. There is only off-loading usage, or with replacement.

You can choose to replace your iPhone 6s with an iPhone with more storage; with a newer or new iPhone.

Do load iOS 13.6.1 though, as that contains some storage-related fixes.

You can't add to the storage capacity on any device.

Delete (enough) stuff from Your iPhone to get more space.