Changing Columns on the Job Dashboard of WP Job Manager

functions.php

<?php

// Customize columns in WP Job Manager [job_dashboard].
function custom_job_manager_job_dashboard_columns( $columns ) {
    // Remove all columns.
    unset( $columns['job_title'] );
    unset( $columns['filled'] );
    unset( $columns['date'] ); 
    unset( $columns['expires'] );
  
    // Add columns with proper headings in the correct order.
    $columns['job_id'] = 'ID';
    $columns['job_title'] = 'Job';
    $columns['date'] = 'Date';
    $columns['job_location'] = 'Location';
    $columns['filled'] = 'Filled';
    $columns['expires'] = 'Expires';
	
    // Return.
    return $columns;
}

// Populate ID column.
add_action( 'job_manager_job_dashboard_column_job_id', function( $post ) {
    echo $post->ID;
} );

// Populate Location column.
add_action( 'job_manager_job_dashboard_column_job_location', function( $post ) {
    echo get_post_meta( $post->ID, '_job_location', true );
} );

Hi,
the question might be "stupid" but I'm really looking for a solution to my problem.
Where should I put (path or folder) this functions.php file in order to get the desired functionality?

Thank you in advance!

Best,
Nas