Q:

How to implement next button and previous button from a list of models in unity?

I have list of 10 models in a list. Initially I have a model  present in my scene say model[0].When I click next button it must show model[1] till model[9].Similarly the previous button in the reverse order.
I have written a logic inside the code.I know it is not a standard one but will do the job I believe.Apart from this logic any simple way to implement. 
public GameObject [] dress;

public void PreviousModel()
{
    int counter = dress.Length;//Dont know what to write here
    Debug.Log(counter);
    if(counter > -1)
    {
        counter--;
        dress[counter].SetActive(true);
        dress[counter+1].SetActive(false);
    }

}

public void NextModel()
{

}

A:

that should do the trick
public GameObject [] dress;

    private int _index;
    public void PreviousModel()
    {
        _index = Mathf.Clamp(_index-1,0,9);
        // code to show your model dress[_index] ...

    }

    public void NextModel()
    {
        _index = Mathf.Clamp(_index+1,0,9);
        // code to show your model dress[_index] ...

    }

A:

Alternatively to the answer using Clamp you could also wrap around the index like
public GameObject [] dress;

private int _index;

public void PreviousModel()
{
    // Hide current model
    dress[index].SetActive(false);

    _index--;
    if(index < 0)
    {
        index = dress.Length -1;
    }

    // Show previous model
    dress[index].SetActive(true);
}

public void NextModel()
{
    // Hide current model
    dress[index].SetActive(false);

    _index++;

    if(index > dress.Length -1)
    {
        index = 0;
    }

    // Show next model
    dress[index].SetActive(true);
}

So that of you click next on the last entry it jumps to the first instead of doing nothing.

if I understand your comment

the index value I have togive the same as the model that is present in scene

correctly you mean that at start of this script you need to get the current index dependend on the currently active model:
private void Start()
{
    // Get the index of the currently active object in the scene
    // Note: This only gets the first active object
    // so only one should be active at start
    // if none is found than 0 is used
    for(int i = 0; i < dress.Length; i++)
    {
        if(!dress[i].activeInHierachy) continue; 

        _index = i;
    }

    // Or use LinQ to do the same
    _index = dress.FirstOrDefault(d => d.activeInHierarchy);
}

