Assuming that Form1 is displaying Form2 then you could have something
similar to the following code.
Form2 f = new Form2();
f.Show();
this.Close();
Be cautioned, however, that if Form1 is the main Form (startup Form) then
closing it will end the application. If this is the case then you may
consider displaying Form2 using its ShowDialog method instead, or you could
hide Form1 instead of closing it.