Here is a quick and easy guide on how to upgrade your Charm
The location of the NPCs that will help you ugprade your items are located in the following areas.
Addon Keep (Human)
Wirtz Keep (Akkan)
Let's break this down in subgroups:
Charm Upgrade
Materials: Pearl & Ancient Pearl
In case you are missing a couple of materials, you may check the farming guide on how to acquire the materials that you need in order to perform the upgrade: Introducing a Beginner's Farming Guide: (Metals, Gems, Ang Pau, Pearls, Etc)
The Charm Craftsman NPCs will assist you in upgrading your charm.