C#

public class frmChangeSettings
{
    //to store a reference to the main form
    private frmTerminal _theTerminalForm;

    public frmChangeSettings(frmTerminal theMainForm)
    {
        this._theTerminalForm = theMainForm;
    }
}

public class frmTerminal
{
    public string TextBox1sText
    {
        get {return this.TextBox1.Text;}
        set {this.TextBox1.Text = value;}
    }
    
    //.... etc for the other properties you want to be able to change
}

this._theTerminalForm.TextBox1sText = "something something"

public class FormSettings : Form
{
    private TextBox txtName;
    
    //...lots of autogenerated code
    
    public string Name
    {
        get {return this.txtName.Text;}
        set {this.txtName.Text = value;}
    }
}

private void settingsToolStripMenuItem_Click(object sender, EventArgs e)
{ 
    Form1 mehmet = new Form1();
    mehmet.Show();
    mehmet.Name = "A name...";
}