To change your class in Phantasy Star Online 2: New Genesis, you must locate the class counter admin inside the central city in the game.
You can find the NPC directly across from the Item lab near the central tower in the game, and to change your class, you will first need to walk up and open up the menu with the class counter admin.
Afterwards, you will be presented with a number of options, but the one you will want to click is the one called change class, as shown below.
Once clicked, you will need to click the option called select class and then pick the class you want to change to, do this now and press ok when prompted and you will be able to change your class in the game this way.
Related: How to get and use Photon Chunks in Phantasy Star Online 2 New Genesis
You can also change your class back or to another one using the same steps as before, and you can even change your skill tree if needed.
After you change your class, you will need to level it up through various methods such as quests, tasks, events, and killing monsters. Then when you get a few level-ups, you will be able to unlock more class points for your new class using the class counter admin again.
For more on New Genesis, check out Phantasy Star Online 2 Comi Codes (June 2021) and How to play with friends in Phantasy Star Online 2 New Genesis