In C#, how do you display a window that already has a previous instance loaded into memory?

Example:
Form1 has a textBox1 and a button. Some text is typed into texBox1. The button click of Form1 loads Form2 which also has a button. Clicking Form2's button calls Form3. How do you call the already running instance of Form1 which contains text in it's textBox1?

Thank you
Roland Thomas