Keep a reference to the tab bar item that you want to modify.
EDIT as a response to your code request:
I believe that there is a single place in your app where you update the badges on the tab bar items. Just add an array of tab bar items (or separate tab bar items) as a member(s) of that class (+ properties if needed) and update the items directly without fetching from the current tab bar items list ((UITabBarItem *)[stTabBarController.tabBar.items objectAtIndex:1];
).
For instance, if you decide to keep references to the tab bar items directly (without an array) then the code might look like that:
// Put the next code right after initiating the tab bar and/or after adding new tab bar items to it...
self.newsTabBarItem = (UITabBarItem *)[stTabBarController.tabBar.items objectAtIndex:1];
self.friendsTabBarItem = (UITabBarItem *)[stTabBarController.tabBar.items objectAtIndex:2];
// etc.