iTunes prefs -> General.
Click the *Import settings* button and set it to AAC.
Select all teh songs then menu Advanced -> Create AAC version.
Then delete the MP3 versions.
Note that converting MP3 to AAC will result in degraded audio quality and larger file size if you use a higher bitrate than the MP3, with no benefit other than they are in AAC format. Anything that will play AAC will play an MP3.
This why I asked, "why" do you want to convert them?
What is the bitrate of the current MP3 files?
What bitrate do you want to convert them to and why?