how to set force not add force in unity code example

Example: Rigidbody.addforce

using UnityEngine;public class ExampleClass : MonoBehaviour
{
    public float thrust = 1.0f;
    public Rigidbody rb;    void Start()
    {
        rb = GetComponent<Rigidbody>();
        rb.AddForce(0, 0, thrust, ForceMode.Impulse);
    }
}