是倒装形式，这本来是从虚拟语气变化而来，不过，现代英语对于这种用法，一般把should 看作和if 相似，通常用于正式文体。请看朗文词典的解释：https://www.ldoceonline.com/dictionary/should
should somebody/something do something Should you need any help (=if you need any help), you can always phone me at the office.
- Should [=if] you change your mind, please let us know. （韦氏高阶例句）
