Class Change Quest
Start from Npc : Ziola (26, -64),
Pokari City - The East Market.
Get and choose your Sub-class
The class change quest is an Ethyr Quest (Found by Available Quests in your Quest Log (Q))
1. Talk to Ziola about quest class change.
You can actually start the subclass quests as low as level 65(60*-75),
But you may not fully complete the class change until level 75 to receive the items given to you.
2. You will have to head to Gallio to meet an Specified NPC
(Mogui - (Hellion and Maven)) head to Zappah Altar for immediate completion
You will have to select the sub-class from 1 of the NPC and it's done.
For the other race and classes (Shenzu, Yaoh, Ren...), you can buy a Class Advancement Scroll (5 E-bucks) to get similar steps to the Mogui race job change, just talk to NPCs and it is completed. Otherwise, the trial continue...
3. You will answer some questions and see NPCs
then you need to wait a short amount of time
then you need to kill 300 mobs
then you need to kill 900 mobs
then you need to kill 1500 mobs
then you will have to do two or three more quest to gather materials (Conjurers must gather 500 of every dust/Shaman must kill Katsune Kid for quest item. Will repeat 10 times/Dragoons must kill Aughorn Kobolts for items and turn quest 5 times/Rangers will have to collect item from Portly Snowhawk and repeat a quest to kill Katsune Kid 5 times/Rogues must kill Katsune kid for quest item 5 times/Mystics must kill Aughorn Kobolds for items and turn quest 5 times.) and to kill in Serpentail Valley in the Gallio Region. The NPC that will ask you to gather material is right next to Serpentail Valley. Be sure to talk to him before you gather the material, or it will not work.
4. Finally you get back to Ziola to complete your trial.
When completed you will receive FREE DEF/RESIS (Defense/Resistance) Destiny level 75 gears (once you complete the task asked) and weapon (when you are fully level 75),
Along the way, you'll also get a free but bound  Topaz Disk to change your sub-class if you do not like the sub-class you've chosen.
Also note that you will need the Topaz Disk for sub-class change at later levels,
since not using a Topaz Disk will definitely ensure that you lose all your spirits and valor during the change.
And to change your sub-class (in the event that you feel you have chosen the wrong one),
Talk back to Ziola with the Topaz Disk, chose the Teleport to -insert NPC which you begin your sub class quest- NPC.
P/S: Just remember to check your quest log (by pressing the alphabet Q) in the game to view quest details and such.
More information available on Sub- Classes (other on forum and in-game guide (L))