Why AAC to MP3, for this AAC file
A car stereo, a cheap speaker, or an old app plays MP3 and ignores AAC.
The MP3 sound you will actually get
The AAC file is encoded as MP3 with LAME in the browser. The first run downloads the converter.
You lose a little more detail. If the player is new enough for AAC, do not convert.
Why AAC to MP3 is its own job
AAC to MP3 sits on this site because AAC and MP3 are not interchangeable, even when they look similar in a file list. A car stereo, a cheap speaker, or an old app plays MP3 and ignores AAC. Apple workflows and some streaming uploads prefer AAC. Podcast hosts, cars, WhatsApp, and old speakers expect MP3.
The AAC file is encoded as MP3 with LAME in the browser. The first run downloads the converter.
The AAC file you already have
AAC is a lossy format used by Apple and many streaming apps. At the same bitrate it often sounds a bit cleaner than MP3. Converting from MP3 to AAC cannot restore what the MP3 removed. It is another lossy generation.
Keep the earlier file. Do not bounce MP3 and AAC back and forth.
What a MP3 result is for
The download has to be MP3, because that is the format the next step named. Podcast hosts, cars, WhatsApp, and old speakers expect MP3. Compatibility is the reason to make an MP3, not an improvement in sound.
Convert once from the best master you have, at the end of the edit.
How to tell that AAC to MP3 worked
AAC to MP3 is worth using only when MP3 is what the next step actually named. A car stereo, a cheap speaker, or an old app plays MP3 and ignores AAC. What you get from AAC to MP3: The AAC file is encoded as MP3 with LAME in the browser. The first run downloads the converter. Before you trust it: You lose a little more detail. If the player is new enough for AAC, do not convert. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. A second pass rarely helps. If the first MP3 file is rejected, read the error from the program that rejected it. Another conversion of a bad result usually makes the file worse, not more compatible. When you are done, you should still have the AAC original and one MP3 download that you have opened. If those two are the same kind of failure, this was the wrong tool and the page above is the place that says so. Audio conversions are one-way once the format is lossy. AAC to MP3 can change the container or the codec so a player opens the file. It cannot make a muffled take clearer, and it cannot turn an MP3 into a studio master by calling the result WAV or FLAC. Finish the edit on the best copy you have, then make the share file once.
Here is the decision for AAC to MP3. You start with AAC and you stop when a real program accepts MP3. It is a good share format when the player accepts it. Each MP3 save throws away a little more. Turning MP3 into WAV or FLAC does not bring the lost detail back. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
You lose a little more detail. If the player is new enough for AAC, do not convert.
A related step, if you still need one, is MP3 to AAC.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.