How do I replace a player torso, for a union in the workspace?

I am making a game, where the player is a cube, and the cube is marked as torso. I want to add skins for the cube, by replacing the torso/cube with a new one in the workspace. I have tried many times but I keep getting errors.

Backround: The union is in a model called StarterCharacter1, I need this to be renamed to StarterCharacter. The union itself is titled Torso. There is a humanoid already in the model I need to move the whole model into the player, and remove the model already there. The model already in StarterPlayer is called StarterCharacter.