Does your mp3 player have a place for an SD card? If it does, transfer your MP3 files onto the SD card in your phone and put the SD card in your mp3 player.
You can plug your phone into your PC and transfer files to an SD card and put the SD card in the mp3 player.
If your mp3 player does not have an SD card you can plug your phone into your PC and transfer the files to your PC and then plug your mp3 into the PC and transfer your files onto the mp3.