How to find the file functions.php
The file functions.php is one of the most important files of a WP theme.
You will find it in all themes and it is in the main theme folder, in example maya/functions.php, or room09/functions.php.
You can find it in two ways.
Via WP Editor:
Navigate to Appearance > Editor.
You will see here a list of files on the right. The file functions.php is named Theme Functions (functions.php)
The list of files may be different from this screenshot. In a child theme there are few files. In a standard theme the list is bigger. But the file functions.php will be always there, in the list.
Connect via FTP to your server. Navigate to wp-content/themes/<your-theme>/. You will find there the file functions.php