How to change Gravity towards certain object in Unity?

There's no need to resort to the gravitational equation. Acceleration due to gravity is constant regardless of mass, so all you want to do is accelerate your small objects towards the large-object each frame.

The code would look something like this:

public void FixedUpdate()
{
    rigidbody.velocity += gravitationalAcceleration * Time.fixedTime * (largeObject.transform.position - transform.position);
}

What others said, gravity on unity is only towards a certain direction, so maybe you should instead disable gravity completly and script a force that moves the small sphere towards the huge one.

In fact, I'm currently working on a game based around gravity: simply, you place stars to move your player through eliptical orbits and the slingshot effect. I disabled gravity, and used this code:

using UnityEngine;
using System.Collections;

public class PlanetGrav : MonoBehaviour {

    //Declare Variables:

//Strength of attraction from your sphere (obviously, it can be any type of game-object)
    public float StrengthOfAttraction;

//Obviously, you won't be using planets, so change this variable to whatever you want
    GameObject planet;

    //Initialise code:
    void Start () 
    {
        //Again, you can change the tag to whatever you want.
        planet = GameObject.FindGameObjectWithTag("Planet");
    }

    //Use FixedUpdate because we are controlling the orbit with physics
    void FixedUpdate () {
        //Declare Variables:

    //magsqr will be the offset squared between the object and the planet
        float magsqr;

        //offset is the distance to the planet
        Vector3 offset;

        //get offset between each planet and the player
        offset = planet.transform.position - transform.position;

            //My game is 2D, so  I set the offset on the Z axis to 0
            offset.z = 0;

            //Offset Squared:
            magsqr = offset.sqrMagnitude;

            //Check distance is more than 0 to prevent division by 0
            if (magsqr > 0.0001f)
            {
//Create the gravity- make it realistic through division by the "magsqr" variable

rigidbody2D.AddForce((StrengthOfAttraction * offset.normalized / magsqr) * rigidbody2D.mass);
            }
        }
    }
}

PS The code originally looped through an array of all the planets: this is edited and therefore may not be fully correct. It should be okay though.