function wc_billing_field_strings( $translated_text, $text, $domain ) {
    switch ( $translated_text ) {
        case 'Billing details' :
            $translated_text = __( 'Billing Info', 'woocommerce' );
            break;
    }
    return $translated_text;
}
add_filter( 'gettext', 'wc_billing_field_strings', 20, 3 );