use itunes..
1 Open iTunes, select Edit then Preferences and click on the General tab. Now click the button that says Import Settings.
2 Change the Import using drop down menu to AAC encoder. Select the bit rate you fancy, have it as high as the highest bitrate MP3 file you want to convert so you a losing as little quality as possible. Hit OK and close the preferences menus.
3 Then in your iTunes Library window select the tracks you want to convert to AAC, an easy way to do this is to press Apple+A or Control+A on your keyboard to select all. If you need to sort then right click the iTunes Kind column header to sort by type.
4 When you have them selected right click on one of the files and choose Convert Selection to AAC and, well, that’s it. Ta Da