Change MySQL 8 root user password

First login and then issue:

ALTER USER 'root'@'localhost' IDENTIFIED WITH caching_sha2_password BY 'yourpasswd';

If you don't know the password or lost it:

sudo systemctl stop mysqld
sudo systemctl set-environment MYSQLD_OPTS="--skip-grant-tables --skip-networking"
sudo systemctl start mysqld

Login (there is no password) and set the password with the above method. FInally:

sudo systemctl stop mysqld
sudo systemctl unset-environment MYSQLD_OPTS
sudo systemctl start mysqld

Test:

mysql -u root -p