Quite easy c: First, find the music you want and make sure it's in .mp3 format. Place these music files in your Media>Music folder, make sure you either write down or remember the names of each music file.
There are then 2 things you can do.
1. Replace the music entirely.
To do this, simply open up the music.cfg found in your main FH folder. You'll replace the names of each music file in there with the names of the ones you put in your music folder.
(http://image.prntscr.com/image/9882cb74a1cd4ba99af18f7f20b2995f.png)
2. Add more music to the existing music
To do this, just copy one of the already existing music lines (like in the screenshot above), and paste it under the others, simply rename the music name to your own.
Be sure to save when you're done, and good luck!