So, when a white button is pressed, remove it and show the same character in the "next" blank position at the top - correct?
One way is to give each Blank an ID instance variable, numbered 0 to 5. Have a Global variable with the current blank position. When the white button is pressed, pick the blank with the current ID, set the text, and increment the position value.
Containers are used to link the Text object to the Sprite.