How to move a GameObject within a Coroutine in C#?

Having two returns in an IEnumerator possibly isn't a good idea (not really sure). I'd have a second coroutine to wait and then start the MoveToTarget and call that coroutine at the end of PlaceLetter. Something like this:

      void Start () {  
          StartCoroutine(PlaceLetter());  
      }
      
      IEnumerator PlaceLetter() { 
          GameObject numberOne =  Instantiate (numberOnePrefab) as GameObject;  
          numberOne.SetActive (true);  
          yield return new WaitForSeconds (1);  
          numberOne.SetActive (false);  
          Destroy (numberOne);
          StartMove();
      }
      IEnumerator StartMove() { 
          yield return new WaitForSeconds (1);  
          MoveToTarget ();  
      }

I haven't looked at your movement script really though. If you are looking to have it move incrementally you might want to put the movement within a coroutine.

Thank you for your reply. Making a second coroutine seems indeed more logical but the letter still doesn't move. I tested the MoveToTarget(),when I put it in the Update () in a script attached to the letterPrefab, the letterprefab moves to the targetPosition. So the MoveToTarget () works. But I don't want to attach a script to the letterPrefab there I'm going to use the letterPrefab in other functions. In my Tutorial script (attached to the Main Camera) I want to get an instance of the letterPrefab and move it the the targetPosition. Is that possible? How? I know that the MoveToTarget() has to be updated every frame and works under Update() in a script attached to the letterPrefab but I don't see how I can implement this in the Tutorial script. Or should I keep the MoveToTarget() attached to the letterPrefab and then when I need the letterPrefab again I disable the MoveToTarget() script?

     using UnityEngine;  
     using System.Collections;
              
      public class Tutorial : MonoBehaviour {  
              
              public GameObject     letterPrefab;  
              public GameObject     numberOnePrefab;  
              public float    speed = 1f;
              
     void Start () {  
                StartCoroutine(PlaceLetter());  
     }
              
     IEnumerator PlaceLetter() { 
               
              GameObject numberOne =  Instantiate (numberOnePrefab) as GameObject;  
              numberOne.SetActive (true);  
              yield return new WaitForSeconds (1);  
              numberOne.SetActive (false);  
              Destroy (numberOne);
              yield return new WaitForSeconds (1);  
              StartMove();  
     }
     IEnumerator StartMove() {    
              yield return new WaitForSeconds (1);    
              MoveToTarget ();  
      }  
 
      void MoveToTarget() {  
  
              GameObject letter = Instantiate (letterPrefab) as GameObject;  
              letter.SetActive (true);  
              Vector3 targetPosition = new Vector3 (-4, 3, -1);  
              Vector3 currentPosition = letter.transform.position;  
              if (Vector3.Distance (currentPosition, targetPosition) > .1f) {  
              Vector3 directionOfTravel = targetPosition - currentPosition;  
              directionOfTravel.Normalize ();  
              letter.transform.Translate (  
                              (directionOfTravel.x * speed * Time.deltaTime),  
                              (directionOfTravel.y * speed * Time.deltaTime),  
                              (directionOfTravel.z * speed * Time.deltaTime),  
                              Space.World);  
              
              }  
     } 
     }