Change the rotation of Z through swipe

I have this code:

 using UnityEngine;
 using System.Collections;
 
 public class PlayerControl : MonoBehaviour {
     public Vector3 startPos, endPos, stayPos, dir;
     public GameObject selectedProjectile;
 
     void Update()
     {
         if (Input.GetMouseButtonDown(0))
         {
             Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
             RaycastHit hit;
 
             if (Physics.Raycast(ray, out hit, 10))
             {
                 selectedProjectile = hit.collider.gameObject;
             }
 
             startPos = Input.mousePosition;
         }
 
         if (Input.GetMouseButtonUp(0))
         {
             Vector3 mousePos3D = Input.mousePosition;
             mousePos3D.z = 10;
             Vector3 worldPos = Camera.main.ScreenToWorldPoint(mousePos3D);
             worldPos.z = 0;
 
             Debug.Log(worldPos);
 
             Vector3 dir = (worldPos - selectedProjectile.transform.position).normalized;
             dir.z = 0;
             selectedProjectile.GetComponent<BasicProjectile>().direction = dir;
 
             selectedProjectile.transform.rotation = Quaternion.Slerp(selectedProjectile.transform.rotation, Quaternion.LookRotation(selectedProjectile.transform.position - worldPos), 10 * Time.deltaTime);
 
             selectedProjectile = null;
             //endPos = Input.mousePosition;
             //dir = (endPos - startPos).normalized;
             //selectedProjectile.transform.LookAt(selectedProjectile.transform.position + dir);
             //selectedProjectile.transform.rotation.z = endPos.z;
             //Quaternion targetQ = Quaternion.Euler(endPos);
 
             //Vector3 relativePos = endPos - transform.position;
             //Quaternion rotation = Quaternion.LookRotation(relativePos);
 
             //transform.rotation.z = Quaternion.RotateTowards(transform.rotation, endPos.z, 0);
         }
 
         if (Input.GetMouseButton(0))
         {
             //Quaternion targetQ = Quaternion.Euler(stayPos);
             //stayPos = Input.mousePosition;
             //selectedProjectile.transform.rotation = targetQ;
         }
     }
 }
 

The selected projectile, which is a cube, is moving with:

 this.gameObject.transform.Translate(direction * speed * Time.deltaTime);

I only want to change the Z axis of the selected projectile, but, the whole rotation changes.