How to change detail page to the other detail page?

I want to change detail page to the other detail page. The other words, when I click the masterpage's button, The detail page will be changed.

Here's my code:

void MyPageClicked(object sender, EventArgs e)
{
new MainPage().Detail = new MyPage();
((MasterDetailPage)Parent).IsPresented = false;
}

'((MasterDetailPage)Parent).IsPresented=false;' is worked, but 'new MainPage().Detail=new MyPage();' code isn;t worked. How do I change the code?