Had assumed this was a read/write property. In that case, you can either follow Dimitris advice and set the style when you instantiate the controller, or (if you're using a XIB), you can set it via IB.
If you have one TableView for more tables, and one of this tables is grouped and the another one plain, than you can simulate the plain style with the function from UITableViewDelegate:
If you're not using Storyboards and just want a clean, concise way to setup your UITableViewController subclass to use a different tableView style, you can simply override loadView like this:
loadView creates the view that the controller manages
it's a method you can ovveride when using ViewControllers whose views are not defined via Storyboards or NIBs.
The UITableViewController implementation already overrides this method for you to crate a simple, plain UITableView as the root view for the controller, but nothing stops you from further overriding it to create a table view that better suits your needs.
Just remember:
Your custom implementation of this method should not call super.