Audacity (free/donateware) has now been around for many years and developed quite a bit. I don't use it (preferring a pro app I have), but Audacity can probably do more than my discontinued app can.

Audacity is available for Windows, MacOSX and Linux. From memory there's also extra plug-in support you can download and install. Check the Quicklinks section for the Manual, plug-ins, etc. Note: There is a learning curve. Especially if you have never used an audio editor before.

Ideally you would save the file to a lossless format (eg: WAV or AIFF) during processing instead of mp3 (eg: make a WAV copy before any processing.) Re-saving as an mp3 will result in a loss of some quality and multiple re-saves will reduce the quality more each time. (Example: Let's say there's 100 dots of information in the mp3 file, if you re-save as an mp3 this might reduce it to 95 or 90 or less dots of audio information. I won't be fussy in discussing how EQ whilst making it sound better can also result in some losses of audio dot information.)

I read where Audacity is commonly used among people in audio file-sharing areas to help edit and process audience recorded live concerts, etc.