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 );