It has been difficult for me (forum is local, used with Wampserver).
I had 2 messages, after having uploaded 3.1.1 to 3.1.2 and started install :
Warning: require(./../phpbb/class_loader.php): failed to open stream: No such file or directory in C:\wamp\www\my forum\install\index.php on line 42
Fatal error: require(): Failed opening required './../phpbb/class_loader.php' (include_path='.;C:\php\pear') in C:\wamp\www\my forum\install\index.php on line 4210:57 26/11/2014
After that, I uploaded full pack and runned
Now it's ok.
I think my english is not good enough to well understand instructions.
I saw in Automatic update package:
Point your browser to the install directory, for example http://www.example.com/phpBB3/install/
Choose the "Update" Tab and follow the instructions