1. Open iTunes, select Preferences and click on Importing.
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.
3. Then in your iTunes Library window select the tracks you want to convert, press Apple+A or Control+A on your keyboard to select all or click the iTunes Kind column header to sort by type.
4.When you have them selected press the Advanced menu, choose Convert Selection to AAC and, well, that’s it. Ta Da.This won’t delete your MP3 files, you will get AAC files alongside your existing MP3 files. Later, if you are happy, you can delete your MP3 files and save some diskspace.
5. If you want to remove all your MP3s after converting to AAC:
6.Firstly make sure you really want to do this, it will save space on your hard disk but personally I would reccomend that you burn all your MP3s to CD/DVD before deleting them.
7. Open iTunes, select Library
8. Right click on a column name (Bit Rate or Name for example)
9. Make sure Kind is ticked as this shows you the file types
10. Click on the Kind column title to sort the file types
11. Left click your first MP3, hold down shift, left click the last MP3 in the list
12. Press deletesource: creationrobot.com