There are programs that you can use OR you can do it easily through itunes if you use itunes. Go to Edit, Preferences, on the General Tap go to Import Settings, another window comes up select import using mp3, setting, select the preferred resolution (higher resolution better sound) and hit Ok
Keep in mind that you are reducing sound quality and most people do not realize you can on some devices store CD Quality music on your ipad, iphones etc. Yes it does take up significantly more space as much as 10 times as much, but if sound quality is important to you maybe you just put your favorite albums at CD quality on your device.
Dont let anyone tell you that mp3 files are just as good and you cant hear a difference, they would be completely wrong.
40 years high end audio video specialist