I will perform a standard phpBB upgrade, this time from version 3.3.4 to 3.3.5.
First of all, be sure to make a backup copy of the files and the database, for example like this:
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 update files:
wget https://download.phpbb.com/pub/release/3.3/3.3.5/phpBB-3.3.4_to_3.3.5.zip
Unpack it and move everything to the forum directory, and if necessary, specify the correct file permissions:
unzip phpBB-3.3.4_to_3.3.5.zip
sudo chown -R www-data:www-data /var/www/ixnfo_com_forum/
After that, the forum will switch to maintenance mode, open the update page in the browser, for example https://ixnfo.com/forum/install/app.php and follow the instructions, the files are updated first, download the proposed archive with the files and unpack it into the forum directory, if there is a notification that you need to delete some files, then we delete them, in the second step, the database was automatically updated for me. Then I removed the install directory and the update process was completed.
See also my article:
How to upgrade phpBB 3.3.3 to 3.3.4