Step one is backup mysql
cd /wiki/mysql_backups ./mysql_backup.sh
Step two, install/upgrade mediawiki
/usr/gnu/bin/tar xvf mediawiki-1.26.2.tar -C /wiki/mediawiki --strip-components=1 chown -R webservd:webservd /wiki/mediawiki
Step three, upgrade db
cd /wiki/mediawiki/maintenance php update.php
Note: You might need to fix the LocalSettings.php