#C (gcc) , <del>20</del>, <del>29</del> 17 bytes
  

    __builtin_bswap32

 [Try it online!][1]

Old answer

    #include<byteswap.h>
    bswap_32



  [1]: 
https://tio.run/##ZcxLCsIwFIXheVZRCpWmtDS9yc2DgisRShtRAnotfeBAdOsxjh0cOHyD3zdX72NbxWGY9nDbAg3T@hxnCbFq2X0MVBJ/XR5L2X9WP9KlzItzXh@I93xeAm0/2LPmmBX7ifKa6r9QCvD@HQXrmALWgU3TyiqZRFuLDoEZoXQCA0waiQ4somNKCGekBSXSBY2yszIpOOW0AYdf