private void change_button_Click(object sender, EventArgs e)
{
     var after_text = before_TextBox.Text?.Select(x=>x.ToString()).ToArray();
     string result = "";
     for (int i = after_text.Length - 1; i >= 0; i--)
     { 
         result += after_text[i];
     }
     after_TextBox.Text = result.ToString();
}

after_TextBox.Text = string.Concat(before_TextBox.Text.Reverse());