Open an Internet browser and navigate to a search engine.
Search for the keyword phrase "mp3 conversion online" without the quotation marks.
Select an online converter that supports conversions from MP3 to AAC.
Locate the file selection field on the chosen web page and select the original MP3 from the folders on the local computer.
Set the original file type as "MP3" if automatic detection is inactive.
Choose "AAC" from the list of output file options and press the "Start" or "Convert" button.
Follow the onscreen instructions to download the converted file to the local computer.