Android : Perform Back instead of Up in Android App

How can I change the behaviour of the "Up" Navigation to the same behaviour as the "Back" button?

My problem is this: I have a navigation drawer in the "MainActivity" which loads various Fragments depending on the selected option. Then I have some other activities when can be accessed using the ActionBar Menu. When I step into one of these Activities and then press back the App navigates back to the PARENT ACTIVITY (which is defined in the manifest as the "MainActivity"). BUT: The fragment which was loaded before is not loaded anymore. It loads the default fragment again.

When I press the back Button in the other Activity it navigates back to the MainActivity properly having loaded the previous chosen Fragment.

Any suggestions on how to solve this?