I recently updated the phpBB 3.3.5 forum to version 3.3.9 on a server with Ubuntu Server, I will describe the upgrade process.
Be sure to back up files and database:
sudo tar -cvjf backup_forum_files.tar.bz2 /var/www/ixnfo_com_forum/ mysqldump -u root ixnfo_com_forum | gzip -c > ixnfo_com_forum.sql.gz
Download the archive with the new version of phpBB and unzip it:
wget https://download.phpbb.com/pub/release/3.3/3.3.9/phpBB-3.3.9.zip unzip phpBB-3.3.9.zip
In the directory with the files of the new version of phpBB, delete config.php, and the directories files, images, store.
In the forum directory, delete everything EXCEPT config.php, ext directories, files, images, store, styles.
Copy the new data to the directory with the forum, docs if desired, set the owner and rights on behalf of which the web server works on the files:
sudo chown -R www-data:www-data /var/www/ixnfo_com_forum/
Update the database by opening the link /install/app.php/update
Select “Update database only”.
If the database is very large, then we will update it from the console:
php ./bin/phpbbcli.php db:migrate --safe-mode
After the update, delete the install directory.
See also my other articles about phpBB