In order to upgrade PHPBB version one need to perform below mentioned steps:
1) Download newest version of PHPBB. [2 zip files will be there…new_version file and old_to_new version file]
2) Disable board from admin panel.
3) Take backup of database.
4) Extract downloaded new_version zip file. Put cache, contrib, docs, install folder into separate folder.
5) Extract another old_to_new version zip file.and put cache, docs, contrib, install folder into the same folder in which we have put the new_version folders. It must be overwrite so any missing file will be replaced.
6) Copy the files on the server.
7) install update via web browser. [/install/update_to_latest.php].
8) delete /install and /contrib folders from the server.
9) enable board from admin panel.