rename multiple files by file.rename or other functions

Hi,

   I have 50 files whose names are

XYZW01Genesis_ABC.mp3
XYZW02Genesis_ABC.mp3
.......
XYZW50Genesis_ABC.mp3

   As you can tell, the only difference across the files are 01, 02,
03,....50.

   I would like to rename them to
01Gen01.mp3
01Gen02.mp3
.......
01Gen50.mp3

  If I store them in one folder and write an R code in that folder, how can
it be done?

   Thanks,

John