Download latest version from https://www.mediawiki.org/wiki/Download
curl https://releases.wikimedia.org/mediawiki/1.34/mediawiki-1.34.0.tar.gz
cd /home/www
tar xvzf mediawiki-1.34.0.tar.gz
cd mediawiki-1.34.0
Copy files from the old version
cp <oldwiki>/LocalSettings.php ./
cp -rp <oldwiki>/skins/<yourskin> skins/
cp -rp <oldwiki>/extensions/<yourextension> extensions/
cp -rp <oldwiki>/images/* images/
chown -R www-data:www-data images
chown -R www-data:www-data cache
Update the database
cd maintenance
php update.php
And bring the new version live
rm wiki
ln -s mediawiki-1.34.0 wiki
sudo systemctl restart nginx
sudo systemctl restart php7.2-fpm