How do I use MariaDB instead of MySQL? I need to use MariaDB instead of MySQL in Drupal 8. I have already installed MariaDB in my local system.

How do I use it with Drupal 8? What changes are required to use it?
MariaDB is a drop-in replacement for MySQL. No changes are required.

You'll obviously need to update your connection/credential details to match the new server, but as far as configuring Drupal to use MariaDB, there's nothing to do.