First, this guide is was made with Windows OS in mind. Some steps may be different for Mac or Linux.
Hello! If you are looking to replace music in I Wanna Maker, you've come to the right place. Let's get started.

First things first, let's find the music files we wish to replace in the game.
To do this, go to your Steam Library.
Right-click "I Wanna Maker". A little tab will pop up with some options.
Click "Properties".
On the left-side of the window that comes up, click "Local Files", then click "Browse", near the top-right of the window.
A file directory will open up some folders.
Locate the folder titled "Music" and double-click it.
Now, we see our music files, most of them ending with the .ogg file format.
Now, we could go about replacing these files, but let's do two things first:

1) Make note of a test track to replace for later: You'll want to get the process of this down for good, so quick feedback to see if what you do works is important. Just find a file for a track that you can easily access in-game first.

2)Create a backup of the original music: Just in case you want all vinilla music in your game. Create a new folder wherever you'd like (where you can easily access it later). Go back to the Music folder and press Ctrl+A and then Ctrl+C. Go to your backup folder and press Ctrl+V. There you go, backup is all done!

Now, take any file you'd like, and make a note of its name.
Now, you will need a music file to put in place of the original.
To do this, you can take whatever music file you have, and convert it into an .ogg file.
(There are several ways you can do this, just look up .ogg converter on Google)
Once you have your .ogg file, you need to re-name this file to the name of the file you intend to replace.
Then, simply put this file into your music folder.
If it prompts you to replace it, say Yes (make sure you made a backup first).
Now, there is only one thing left to do!

Now all you need to do is load up your game, and go to where you can easily hear your music being played.
If your hear your song come through, good job, you just replaces the music!
If you hear the standard song, check that the name of your track is identical to the one you chose to replace.

Thank you for reading my guide.
Note that I only know how to replace existing, not add whole new tracks to the game.
If you have any questions or suggestions to make this guide better, please leave a comment.
Have a great day!