Miraculously, I’ve managed to find the password written down in my notes. So, I was able to successfully upgrade.
I have decided to follow the guide in your link to change it.
sudo systemctl stop mariadb
sudo mysqld_safe --skip-grant-tables --skip-networking &
In a seperate tab:
sudo mysql -u root
MariaDB [(none)]> use mysql
MariaDB [mysql]> flush privileges;
MariaDB [mysql]> ALTER USER 'root'@'localhost' IDENTIFIED BY 'the_password_i_want';
MariaDB [mysql]> exit
The command:
kill $(cat /var/lib/mysql/$HOSTNAME.pid)
didn’t work for me and I had to run:
ps -aux | grep my
Then:
kill -9 <PID>
for every PID number on the list (I screwed up opening the server and there were three instances running and it wouldn’t allow me to restart mariaDB).
Finally,
sudo systemctl start mariadb
Thank you so much for your help.
Sometimes you just can’t function if you’re afraid to screw up something, that’s why it’s good to ask. 
