In the editor, click on the hero, look at the left panel of the editor, find the Shared line there, and click on "...". Then, among the lines, you need to find Specialization and select the desired one from the drop-down list. To change the name, image, and other things, you need the parameters SpecializationName, SpecializationDesc, and SpecializationIcon. The names might be slightly different; I don't remember them exactly right now, but they should be similar.
I did it that way, but the problem is that after the game starts, the skill doesn't change and remains the same as it was before.