When you try to run Magento 2 and If you get this error message, possible reason is wrong username or password.

You can change this by editing, below information.

Go to app folder > Etc > and open env.php

In this file, you can find below code section

array (
'host' => 'localhost',
'dbname' => 'magento2',
'username' => 'root',
'password' => '',
'model' => 'mysql4',
'engine' => 'innodb',
'initStatements' => 'SET NAMES utf8;',
'active' => '1',
)

Replace your database information.