以下のコードを使用して、CGRectMake(0、0、100、100)を好きな位置に変更します。
FirstViewController *firstViewController = [[FirstViewController alloc] initWithNibName:@"FirstViewController" bundle:nil];
UINavigationController *modalNavigationController = [[UINavigationController alloc] initWithRootViewController:firstViewController];
UIPopoverController *popoverControllerTemp = [[UIPopoverController alloc] initWithContentViewController:modalNavigationController];
self.popoverController = popoverControllerTemp;
[self.popoverController presentPopoverFromRect:CGRectMake(0, 0, 100, 100) inView:self.view permittedArrowDirections:UIPopoverArrowDirectionAny animated:YES]