How to set gameobject dynamically in script?

I am a newbie in unity i don't even know if my question is very clear but please try to help me im really trying hard anyway. I am making a checker like game so i already have a board and a checker piece that can move anywhere on the board but my problem is my gameObject selectedUnit is fix and i don't know how to dynamically change it in run time. This is my code.

public GameObject selectedUnit; //<--------------This is my problem i think I want this to be assign depending on what checker piece i click on the board how can i do that?

 public TileType[] tileTypes;
 int[,] tiles;
 int boardSizeX= 8;
 int boardSizeY= 8;

 void Start() {
     GenerateBoardData ();
     GenerateMapVisual();
 }
 void GenerateBoardData(){
     //Declare board size.
             tiles = new int[boardSizeX, boardSizeY];

     //Initialize board tiles
     for(int x=0; x < boardSizeX; x++){
         for(int y=0; y < boardSizeY; y++){
             //Initializing the color of the board.
             if ((y % 2) == 0) {
                 if((x % 2) ==0)
                     tiles [x, y] = 0;
                 else
                     tiles [x, y] = 1;
             } else {
                 if((x % 2) ==0)
                     tiles [x, y] = 1;
                 else
                     tiles [x, y] = 0;
             }
         }    
     }
 }

 //This code is just for displaying the board for the game.
 void GenerateMapVisual(){
     for(int x=0; x < mapSizeX; x++){
         for(int y=0; y < mapSizeY; y++){ 
             TileType tt = tileTypes [tiles [x, y]];
             GameObject go = (GameObject)Instantiate( tt.tileVisualPrefab, new Vector3(x, y, 0), Quaternion.identity);
             TileClickHandler ct = go.GetComponent<TileClickHandler> ();
             ct.tileX = x;
             ct.tileY = y;
             ct.map=this;
         }    
     }
 }
 public void MoveSelectedUnitTo(int x, int y){
     selectedUnit.transform.position = new Vector3 (x, y, 0);
 }
 

I think i need to make a script for all of my pieces but i dont know what i should i do please help.