C# Pong: How to increase ball speed over time?

So im currently working on a simple Pong game in Unity. I’ve got everything working out fine so far but I can’t seem to figure out how to get the ball speed to increase over time. Here’s my script which is attached to the ball:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ballMovement : MonoBehaviour
{
    public float speed = 10;

    void Start()
    {
        // Initial Velocity
        GetComponent<Rigidbody2D>().velocity = Vector2.right * speed;
    }

    float hitFactor(Vector2 ballPos, Vector2 racketPos,
                float racketHeight)
    {
        // ascii art:
        // ||  1 <- at the top of the racket
        // ||
        // ||  0 <- at the middle of the racket
        // ||
        // || -1 <- at the bottom of the racket
        return (ballPos.y - racketPos.y) / racketHeight;
    }

    void OnCollisionEnter2D(Collision2D col)
    {
        if (col.gameObject.name == "RacketLeft")
        {
            // Calculate hit Factor
            float y = hitFactor(transform.position,
                                col.transform.position,
                                col.collider.bounds.size.y);

            // Calculate direction, make length=1 via .normalized
            Vector2 dir = new Vector2(1, y).normalized;

            // Set Velocity with dir * speed
            GetComponent<Rigidbody2D>().velocity = dir * speed;
        }

        // Hit the right Racket?
        if (col.gameObject.name == "RacketRight")
        {
            // Calculate hit Factor
            float y = hitFactor(transform.position,
                                col.transform.position,
                                col.collider.bounds.size.y);

            // Calculate direction, make length=1 via .normalized
            Vector2 dir = new Vector2(-1, y).normalized;

            // Set Velocity with dir * speed
            GetComponent<Rigidbody2D>().velocity = dir * speed;
        }
    }

}

what worked for me was (assuming the ball is already moving), I have inside an Update():
rb.AddForce(rb.velocity x speed x Time.deltaTime)