MeshTestBend.cs

using UnityEngine;

public class MeshTestBend : MonoBehaviour
{
    // We will get 2 copies of vertices,
    // so that we could use the original positions as an input
    Vector3[] vertices;
    Vector3[] originalVertices;

    // Reference to the mesh
    Mesh mesh;

    void Start()
    {
        var meshFilter = gameObject.GetComponent<MeshFilter>();
        mesh = meshFilter.sharedMesh;
        
        // get 2 copies of vertices from an existing mesh
        vertices = mesh.vertices;
        originalVertices = mesh.vertices;
    }

    void Update()
    {
        for (int i = 0; i < vertices.Length; i++)
        {
            // Move vertices in a sine wave according to their starting positions and time
            vertices[i].y = originalVertices[i].y + Mathf.Sin(originalVertices[i].z + Time.time);
        }

        // Assign vertices to mesh
        mesh.vertices = vertices;
    }
}