Please don’t do that. There is absolutely no way to upconvert an audio track. Once it was compressed down to anything below 320, that data is lost forever. The best thing you can do with your tracks that are lower than 320 is to leave them alone or try to find new, higher bitrate versions.
If you were tech savvy, you could try looking into iTunes match, as I had great luck replacing many of my lower bitrate songs with updated versions of them but I don’t think that tedious trial-and-error approach will work for most people.
Finally, I would recommend lossless in the form of flac or m4a for any new tracks you rip but always remember that once a song has been compressed into a lossy format, that information is lost forever. You should google around for explanations of lossy and lossless compression.
Peace