Update for Slackware x64: latest version of MariaDB 10.0. This package was build on a Slackware 14.1 system. To build this package succesfully on a 14.1 system, you also need to install the jemalloc package.

 
Downloads:

• MariaDB 10.0.25
• jemalloc 3.6.0

How to upgrade from MariaDB 5.5.x to MariaDB 10.0.x

 
1. Start by downloading the 2 packages above.
 
2. Transfer these packages to the server you want to upgrade.
 
3. First install the jemalloc package:

installpkg jemalloc-3.6.0-x86_64-1.txz

 
4. Stop the running mysqld service:

/etc/rc.d/rc.mysqld stop

 
5. backup ALL your databases (make a copy of the entire /var/lib/mysql directory).

cp -a /var/lib/mysql/ /var/lib/mysql-BACKUP

 
6. Uninstall the MariaDB 5.5.x package (use removepkg or pkgtool).
 
7. Install the new MariaDB 10.0.25 package:

installpkg mariadb-10.0.25-x86_64-1.txz

 
8. Make sure /etc/rc.d/rc.mysqld is updated (overwrite the old rc.mysqld with rc.mysqld.new).
 
9. Start the mysqld service:

/etc/rc.d/rc.mysqld start

 
10. Check if mysqld is running now:

ps aux | grep mysql

 
11. Run mysql_secure_installation. Maybe this is not really necessary, but it doesn’t hurt to start clean. While you’re at it, you can set a new mysql root password here as well.
 
12. Run mysql_upgrade (MUST DO):

mysql_upgrade -u root -p

 
10. Restart the mysql service again (just to be sure).

/etc/rc.d/rc.mysqld restart

 
11. Now go check your DB’s and applications.

Enjoy 🙂
 
EDIT 27-Oct-2019: The above procedure is also valid when upgrading from say MariaDB 10.0 to 10.2 or later versions.