mysqld_safe --skip-grant-tables &
update mysql.user set authentication_string=password('Root_1234') where user='root';
flush privileges
killall -TERM mysqld

systemctl start mysqld
then, mysql -uroot -pRoot_1234
ALTER USER 'root'@'localhost' IDENTIFIED BY 'Root_1234';
flush privileges;

or else, it will report error, you must execute alter user before you use it .