I created a variable for myTestActor in MyCharacter class and I want to access one of myTestActors in the scene but I am trying to change it in the defaults panel and it wont change.
I believe this cannot be done in the default, because the behavior of the default is to be the same in EVERY scene. And your actor will not be in every scene. You cannot reference a Level-Specific actor in the generic class values, it does not make any sense!
You should use EditInstanceOnly instead, and have your reference set only on an instance of your object.