How do I rotate an instantiated prefab?

Hello !

Iv'e made a script so I can instantiate a prefab at my current mouse position for my building game.
But right now the prefab only have one rotation and when I press "R" I want to rotate it 90 deg.
This is my current code for instantiating a prefab.

Code (CSharp):

1.     private Transform currentBuilding;
2.     private Grid grid;
3.     void Awake()
4.     {
5.         grid = FindObjectOfType<Grid>();  
6.     }
7.  
8.     void Update()
9.     {
10.         if (currentBuilding != null)
11.         {
12.             Vector3 m = Input.mousePosition;
13.             m = new Vector3(m.x, m.y, transform.position.y);
14.             Vector3 p = grid.GetNearestPointOnGrid(GetComponent<Camera>().ScreenToWorldPoint(m));
15.             currentBuilding.position = new Vector3(p.x, 0, p.z);
16.         }
17.  
18.     }
19.  
20.     public void SetItem(GameObject b)
21.     {
22.  
23.         currentBuilding = ((GameObject)Instantiate(b)).transform;
24.  
25.     }

Any ideas on how I can rotate the instantiated prefab?

 

Try this?
https://docs.unity3d.com/ScriptReference/Transform.Rotate.html
and
https://docs.unity3d.com/ScriptReference/Input.GetKeyDown.html