If from some reason reset fails follow the step below to use –skip-grant-tables to reset MySQL root password.
Let’s start by stopping the currently running MySQL database:

$ sudo service mysql stop

Next, create a /var/run/mysqld directory to be used by MySQL process to store and access socket file:

$ sudo mkdir -p /var/run/mysqld
$ sudo chown mysql:mysql /var/run/mysqld

Once ready manually start MySQL with the following linux command and options:

$ sudo /usr/sbin/mysqld –skip-grant-tables –skip-networking &
$ [1] 2708

Confirm that the process is running as expected:

$ jobs
[1]+ Running sudo /usr/sbin/mysqld –skip-grant-tables –skip-networking &

At this stage we are able to access MySQL database without password:

$ mysql -u root
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.7.20-1ubuntu1 (Ubuntu)
Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type ‘help;’ or ‘\h’ for help. Type ‘\c’ to clear the current input statement.

mysql>

Using the MySQL session first flush privileges:

mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.00 sec)

Next, reset root password. The following commands will reset MySQL root password to linuxconfig.org:

mysql> USE mysql;
Database changed
mysql> UPDATE user SET authentication_string=PASSWORD(“linuxconfig.org”) WHERE User=’root’;
Query OK, 0 rows affected, 1 warning (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 1

mysql> UPDATE user SET plugin=”mysql_native_password” WHERE User=’root’;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0

Quit MySQL session:

mysql> quit
Bye

Gracefully terminate current mysqld process:

$ sudo pkill mysqld
linuxconfig@ubuntu:~$ jobs
[1]+ Done sudo /usr/sbin/mysqld –skip-grant-tables –skip-networking

Lastly, start MYSQL database:

$ sudo service mysql start

Bron: https://linuxconfig.org/how-to-reset-root-mysql-password-on-ubuntu-18-04-bionic-beaver-linux