/app/etc/env.php
Look for the following:-
'db' =>
array (
'table_prefix' => '',
'connection' =>
array (
'default' =>
array (
'host' => 'localhost',
'dbname' => 'DB_NAME',
'username' => 'DB_USERNAME',
'password' => 'DBPASSWORD',
'active' => '1',
),
),
),
Change DB_NAME, DB_USERNAME and DBPASSWORD to the new MySQL Database that you want to use.