Talk to your new class trainer in which ever inner city zone of Freeport they are.
If you are a neutral class, then you can choose whether to stay your current class or switch to the other associated class.
If you are an alignment restricted class, then you must talk to the trainer for opposite alignment to your class.
You will be asked several times if you are sure you want to be your class to confirm your choice.
NOTE: Your hotbars will reset after you confirm your choice, even if you do not change classes. Before confirming, you may want to save your hotbar configuration, as described in the main Betrayal article.