A reader wants to convert .mp3 music files to the .aac format.
Before I say how, let me ask "Why?" There's only one good reason I can think of: You just got a Nintendo DSi. That's the only digital music player I know of that supports .aac files but not .mp3s.
Otherwise, don't do it. If you want to take advantage of AAC's superior sound quality, you'll be disappointed. The information that was lost when the audio was converted to MP3 cannot be regained by conversion. In fact, sound quality could actually get worse, because your music will have gone through two separate lossy compressions rather than just one.
But if you have a DSi:
You have two options. If you've got iTunes, simply select and right-click a bunch of songs and select Create AAC Version. That will do it.
If you don't have iTunes, and don't want to install it for this one-time job, download and install The Format Factory. This free program (paid for with moderately annoying advertising) can convert just about any audio and video format into any other.