I want to change my hero to a raider how do I do this?

If you have a Class change item or are at a level where you can change class and you aren't being prompted to choose a class then you can tap on your Main base building and select the Class option. You will see the different classes you can choose from.

If you don't have a class change option then keep leveling up as you get a chance to change every few base levels and you may also be able to buy a class change.