Just to clarify some points to give the correct advice
The downloaded file is an mp3?
You need the trimmed song to be in m4a?
If that positive for both, then some form of transcoding is inevitable, and since mp3 is a lossy format, and m4a is normally a lossy format as well, you might encounter some quality loss depending on how high the mp3 bitrate is compared to the m4a. You can select an higher bitrate for the m4a, but you’ll never achieve an higher quality than what the mp3 contained. For that situation, a relatively easy and visual software to do so would be Audacity.
If you are able to keep the trimmed song as an mp3, then there are some utilities like mptrim that pick the portion of audio you want without retranscoding, which avoid degrading the signal.