Modification involves using a method called 'Reforming' to add various stats to your permanent costumes. I've made a small step by step guide to help show the process of leveling them.
Premium Silk can be obtained through Mai Golai in eversun city. Or bought through other players. Mai Golai will take the following costumes and give in return silk.
Body perm = 25 silk
Head perm = 15 silk
Face perm = 10 silk
Back perm => does not accept these
Insurance Scrolls:
If you plan on not wanting to lose your costume, you might want to invest in these during the modification process. There are different types of Insurance Scrools so be careful to read the details.Insurance Scroll (Mod) I will prevent your costume from disappearing if a reform fails, but your costume's current level will lower by 1, and the materials you used will have been lost.
Modifying The Costume:
Click the Skills button (Ctrl - X) and click on the Everyday Skills tab. Click on the button that says "Mod" or "Modify", with the image of a hammer and shirt. Drag the costume you want into the upper left slot. When doing so, the bottom slots will reveal the mats needed and whether or not you have them. The upper right slot can hold the insurance scroll, to prevent the death of your costume.
Some things to note:
*A costume can be reformed to a level of 20. When you reach level 20 a system message pops up that everyone can see.
*As the costume's level goes up, there is a greater chance of failure
*Failure can result in level decrease or complete destruction of the costume. Using an insurance scroll just ensures that only level decrease will happen if the reform fails.
*Most costumes show the level of reform they are, but there are a couple that don't
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