change bitrate of mp3 with ffmpeg

Hi,
Can someone tell me how to change the bitrate of a mp3 file with ffmpeg? I tried the following command:

ffmpeg -i myfile.mp3 -b 64k output.mp3

but I always have an I/O error (the error says: "usually that means that input file is truncated and/or corrupted").

Any hint appreciated
geantbrun