I am making a 3D game with Multiple levels.
Some levels have different Camera positions & Camera follow scripts.
So, I am not instantiating Character but instead have Character GameObject Prefab in each and every Scenes itself.
In Main Menu I made the option to change characters.
So, how do I make the Characters in the levels change into same as the Selected Character in Main Menu?
If I understand it correctly you want information to get saved between scenes. You choose a certain character in a scene (MainMenu) and then you want every scene to use that character.
To do that you could use a class that stores the character selected and prevent it from being destroyed when you load another scene. You can do that with DontDestroyOnLoad. Now you will have an object that never will get deleted unless you manually choose to do so. The next issue is to access this object, I would use something like a singleton pattern to make it available from anywhere. Then you could acess the character variable in some sort of start function and change it.