Apple guidelines for back button
It is specified in the iOS Human Interface Guidelines that there is a default back button positioned in the top left. In full, it reads:


  A navigation bar appears at the top of an app screen, below the status bar, and enables navigation through a series of hierarchical app screens. When a new screen is displayed, a back button, often labeled with the title of the previous screen, appears on the left side of the bar. Sometimes, the right side of a navigation bar contains a control, such as an Edit or a Done button…


More information can be found in the API documentation on the UINavigationBar.