At level 40 you may leave your current class and begin again as another. You reset to level 1. Most skills and spells stay on your sheet, but much of it will not work until you earn the levels back in the new class.
Your first remort is straightforward: go to your class master and say multi with the class you want. No quest, no token.
Once you have taken all three classes to 40—a hero—every remort through that master requires the long class quest and the crafted token. Your gear is stripped when the ritual succeeds.
Open the guide for the class you are leaving:
First remort
Hero
In-game rules: help multiclassing. Class background: the three classes.