how to make a button make an image show inside another button

I am working on a menu for my game. It has a list of buttons with all my “companions” pictures inside the buttons. I have another button at the top of my screen that is the unlock button. When you click on a companion within the list of buttons, i want that companion to appear in the unlock button, and then when the player clicks the unlock button, it unlocks the companion for him.

The companions are .fbx files, the button is a square with the head of my companion inside of it. It is an onclick runtime only button.

Any help at all would be wonderful.
Thank you,

Do you already have a Sprite for each Champion?

You could add an Image game object on top of your unlock button.

When you click on a champion button, update the sprite of the unlock buttons image.