Stop velocity at the position you want

Hi there!
I am a noob coder who is interested in game development. Trying to make an infinite runner game but the character is a bird so there isn't gravity. I have searched the forum and on the google, i couldn't find the solution or didn't use the right words to search.

I have this script on a bird that makes bird always goes to forward.

Code (CSharp):

1. void FixedUpdate() {
2.         rigidbody2D.velocity = new Vector2 (forwardSpeed, rigidbody2D.velocity.y);
3.     }

If i use this code transform.position = new Vector2(transform.position.x, 4f); to change the position of bird, it goes instantly there. But i want it to go smoothly like velocity. If i use this rigidbody2D.velocity = new Vector2 (forwardSpeed, forwardSpeed); to move bird up and down, i can't stop the bird at the position that i want.

Code (CSharp):

1. int yer = 1;
2. void Update () {
3.  
4.         if (yer == 0 && Input.GetKeyDown (KeyCode.UpArrow))
5.         {
6.             Debug.Log ("You are at top");
7.         }
8.         else if (yer == 1 && Input.GetKeyDown (KeyCode.UpArrow))
9.         {
10.             yer--;
11.             transform.position = new Vector2(transform.position.x, 4f);    
12.         }
13.         else if (yer == 2 && Input.GetKeyDown (KeyCode.UpArrow)) {
14.             yer--;
15.             transform.position = new Vector2(transform.position.x, 0f);
16.         }
17.         else if (yer == 0 && Input.GetKeyDown (KeyCode.DownArrow)) {
18.             yer++;
19.             transform.position = new Vector2(transform.position.x, 0f);
20.         }
21.         else if (yer == 1 && Input.GetKeyDown (KeyCode.DownArrow)) {
22.             yer++;
23.             transform.position = new Vector2(transform.position.x, -4f);
24.         }
25.         else if (yer == 2 && Input.GetKeyDown (KeyCode.DownArrow)) {
26.             Debug.Log("You are at bot");
27.         }
28.     }

Thanks for your help!