Custom translation file (Not using Locales/Multisite)
You are looking for Static Translations.

You can add a translation file for English to English (or whatever language you are using) to override default translations. In your case, you'll want to override the app translation category:

In your project folder:

website.com/
└── translations/
    └── en/
        └── app.php


And in app.php:

&lt;?php

return [
    'Invalid username or password.' =&gt; 'Something new'
];