$ sudo mysql -u root -p
Now you can add a new MySQL user with the username of your choice.

mysql> CREATE USER 'newuser'@'localhost' IDENTIFIED BY 'password';
And finally grant superuser privileges to the user you just created.

mysql> GRANT ALL PRIVILEGES ON * . * TO 'newuser'@'localhost';

after that go and edit .env DB_USERNAME=newuser
DB_PASSWORD=password