How to upgrade my storage

Welcome, tebogo256, to Apple Support Communities!

You are asking about «upgrad[ing] [your] storage».

Are you asking about upgrading your iCloud storage?

Or are you asking about increasing the internal, hardware storage of your iDevice (iPhone 6s Plus?)?

These is no way to increase the size of the internal, hardware storage of your iDevice. What it came with, in the beginning, is what it will always have.

However, you can use your iCloud storage to offload some files from your iDevice, or, at least, substitute smaller versions, on your iDevice, while retaining the full versions on iCloud.

This is talked about in Manage your photo and video storage - Apple Support.

I also recommend you follow the steps in Clear Other storage on your iPhone or iPad - Apple Support.

You can not upgrade storage on an iDevice.

If instead you wish to increase cloud storage, see: Buy more iCloud storage - Apple Support