Character Class is created in c++ code and Capsule is Inherited variable (wich means it is defined in one of parent classes), so you can not manipulate it by simply dragging and replacing it. I’m not sure if you can change it without getting into code at all.
So like the previous commentor said, you would have to create your own character class, including all the needed components and instead of capsule add other shape. But there’s another problem, as far as I know movement component relies on collision to be capsule, so that might be a problem too.
Why do you need any other collision shape? You might just add another collision inside your character and attach it to capsule, make capsule small and see what happens