So in order to to change the plugin back to mysql_native_password:

1. Login with sudo:

   sudo mysql -u root
   

2. Change the plugin and set a password with a single command:

   ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'test';
   

note test is your password

OR 

ALTER USER [email protected] IDENTIFIED VIA mysql_native_password USING PASSWORD("verysecret")