function custom_checkout_text_change( $translated_text, $text, $domain ) {
    if ( $domain === 'woocommerce' && $text === 'Billing details' ) {
        $translated_text = 'Billing Details';
    }
    return $translated_text;
}
add_filter( 'gettext', 'custom_checkout_text_change', 20, 3 );