How to reset mysql root password for Mysql Server 5.7 on Ubuntu

Note: the following instruction will DELTE all existing data in Mysql.

1.  Remove existing Mysql:
apt purge mysql-server-5.7 mysql-server-core-5.7 mysql-common

2. Remove existing data:
mv /etc/mysql/ /etc/mysql.bak # or rm -rf /etc/mysql
mv /var/lib/mysql /var/lib/mysql.bak # or rm -rf /var/lib/mysql

3. Re-install mysql
apt install mysql-server-5.7 mysql-server-core-5.7 mysql-common
During this step, the installer will ask you for new root password.

[done].