public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        panel1.Controls.Add(new Screens.UC1());

    }

    private void button1_Click(object sender, EventArgs e)
    {
        foreach (Control ctrl in panel1.Controls)
        {
            ctrl.Dispose();
        }
        panel1.Controls.Add(new Screens.UC1());
    }

    private void button2_Click(object sender, EventArgs e)
    {
        foreach (Control ctrl in panel1.Controls)
        {
            ctrl.Dispose();
        }
        panel1.Controls.Add(new Screens.UC2());
    }
}

public partial class UC1 : UserControl
{
    public UC1()
    {
        InitializeComponent();

    }

    private void button1_Click(object sender, EventArgs e)
    {
        //Event to change text on UC2
    }
}

public partial class UC2 : UserControl
{
    public UC2()
    {
        InitializeComponent();
    }

    public void WriteText(object sender, EventArgs e)
    {
        label2.Text = "Text Changed...";
    }
}