One option is to use MP3-to-WAV converter and first convert this file to WAV, then useing WAV-to-MP3 converter, set bit rate to 128bps and convert it back to MP3 ..
There are lots of programs, but i suggest you
You can simply use the everywhere programs, like JETAUDIO.
When saving As.. you can chose the bitrate.
If you haven't jetaudio, tell me.
Cheers.