To change your class, you have to first get the abilities. To see what abilities you need, right click on any hardline and select "Abilities". Look in the tree you want and see what you need. You can go as far down as your memory permits. Once you've got the abilities, you just upload it and then you can select it in that same "Abilities" panel. That panel is where you change any of your abilities you have loaded. Hope this helps. 
