Athene, the Guardian of the Gods of Madrigal, has taught the blacksmiths in Flaris a new forging technique. With this technique, the blacksmiths can upgrade your Fashion Equipment (CS) all the way to +10!
However, upgrading requires special materials from a different dimension - the Golem Temple. These materials can also be obtained from the Vote Shop and Guild Siege Manager.
To protect your Fashion from failure during upgrading, you can purchase the Scroll of CProtect from [Cash Shop] Laura in Flaris, rather than having to farm for it.
At upgrade levels +4, +6, +8, and +10, you can unlock jewel slots for your Fashion parts. By placing jewels in these slots, you can receive random stat boosts.
If you ever want to remove a jewel from your Fashion, you can use the Scroll of Gem Reversion to do so.
It's worth noting that if you merge an upgraded Gaia Fashion with an Ultimate Fashion, the upgrades will still be present in the final product.