How to change texture format in Unity 5.5.0?

I used to change the textrue import format as what I want in Unity 5.4 and previsou versions. However, after upgrade the Unity 5.5 the Format options has been locked as Auto.

I tried lots of the combinations of the other options to try to make this Format to be editable but no luck. And the documentation aslo metioned this TextureImporter.textureFormat has been deprecated.

Do I have to use Auto for my textures? Do I ever have a chance the change it as what I want in Unity 5.5?

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You have this Default button on top, click this on right of it for pc settings.