how to refresh or update MainForm from Form2

Use delegates and raise an event from Form2. Have your main form subcribe to the event and perform the refresh

public partial class Form1 : Form
{
    Form2 form2;
    public Form1()
    {
        InitializeComponent();
        form2 = new Form2();
        form2.FormDataSaved += new Form2.OnDataUpdated(form2_FormDataSaved);
    }
    void form2_FormDataSaved()
    {
        this.Refresh();
        // However you want to update this form
    }
}

public partial class Form2 : Form
{
    delegate void OnDataUpdated();
    public event OnDataUpdated FormDataSaved;
    public Form2()
    {
        InitializeComponent();
    }
    private void SomeSaveOperation()
    {
        // Whatever your save code is
        this.FormDataSaved();
    }
}

Updating one form from another makes me feel a little queasy to be honest. What you're doing is tightly coupling them together and this approach tends to lead to monolithic applications where you're ability to reuse things becomes limited.

You normally have a third thing (some sort of dataset in you case?) which acts as a shared component. Then, one form updates the the dataset and the other responds to it automatically. This way you can then use either form without the other.