How to set a form as the Main Loading form Windowsforms

I have one form let's call it registration which i created first, I now have a second form lets call it menu. I know these were created the wrong way around but now when i run my Win-forms application it only ever loads the registration form , i am trying to switch my Menu form to be the main loading form (to which i will add a function/button to then load the registration form). I have tried looking in the panel to perhaps set a parent/child relationship cant but find anything

In the Program.cs there is a method called Main.

[STAThread]
static void Main()
{
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);
    Application.Run(new Form1());
    //                  ^^^^^
}

Change the Form1 to the other form you wish to create at startup.