Changes on a item of the main thread usings events

Hello,

i am having problems to change items of the view when i hook an event.

I have this method in the current page:

private async void DoAction()
{
       PageModel.DoSync ();

}

And in the view model:

public void DoSync()
{
         _sincronizator.DoSync();                       
}

When _sincronizator.DoSync() finishes launch an event who is hooked on the current page and fires another one.

void Completed (object sender, bool result)
{
        if (result) { lblInfo.Text = "Sync Completed"; }
}

The app crashes when execute this line lblInfo.Text = "Sync Completed";so I can´t modify items on the current page.

Can you help me, please?

Thanks a lot in advance!