Make Cube flip on its side on GetKeyDown input C#

I want the player (cube) to flip on its side in 2 directions when pressing A and D. Right now i have a really basic script for the movement just to test things out. I'm having trouble finding anything which will allow the cube to smoothly flip on its side quickly when moving left and right instead of just shifting there with the script i have now, i hope this makes enough sense, i'm not to experienced with unity so its difficult for me to get more specific

Code (CSharp):

1.   if (Input.GetKeyDown("a"))
2.         {
3.  
4.             transform.Translate(0, 0, movespeed * Time.deltaTime);
5.          
6.  
7.         }
8.  
9.         if (Input.GetKeyDown("d"))
10.         {
11.             transform.Translate(0, 0, -movespeed * Time.deltaTime);
12.  
13.         }

Blue cube being the player, the flip would go to the left and right of this platform at a fixed distance, so there are 3 possible "paths" to be moving on. So when press A to flip left, what would be the left side of the cube, would be the side facing the ground and vise versa.

 

Where are you putting this code? in update() or else?
search in the forum probably there is more than one answer
https://forum.unity.com/threads/rolling-a-cube.94417/