how to change my subclass
If you are referring to your role (Warrior, Chi Kung, Holist, Strategist), you are able to change your role once after selecting it by speaking to a particular NPC in your clan base. This method only works before level reaching Five Dragons (level 61).
If you are past this level already, the only method of changing your role is by consuming a Role Change Pill, which is only available from the Papaya Marketplace. The item is tradable so you may be able to find someone who will sell you one, but expect to pay a lot.