How to convert MP3 to AAC
http://www.softe.net/eufony_help/convert-mp3-aac.php

AAC, Advanced Audio Coding, is a technique for compressing digital
audio files. Officially part of the MPEG-4 standard, it is most widely
used to create small digital audio files. You might find ringtones in
AAC format in your mobile phone.

Follow the steps below to convert MP3 files to AAC.

Converting MP3 to AAC using Eufony MP3 AAC Converter
http://www.softe.net/audio_converter.php

1. After launch Eufony, In the folders pane, click on the folder
contains MP3 files you want to convert to AAC.
2. In the file list pane, select one or more MP3 files.
3. Right click on the selected MP3 files to bring up the context menu
and select Convert.
4. In the Audio Format Conversion window, select AAC from the Output-
>Format list.
5. If you want to change the output quality, select the Custom radio
button. (optional)
6. If you want to change the destination folder for AAC, click on the
Browse icon button to change. (optional)
7. Click Convert to start converting.