How do you change the code?

To change the code use some sort of text editor IDE, batch text processor whatever you like. (some of these options might be useless in most cases)

Since I assume you actually want know how to download, build, etc:
https://github.com/cabaletta/baritone/blob/master/SETUP.md#build-it-yourself and use an editor or IDE of your choice to edit the code in /src before running gradlew build / gradlew build -Pbaritone.forge_build / gradlew runClient (command line) or double clicking build (IntelliJ).
And just in case you want to edit the code in this repo and not locally:
Use the fork button on the top right of your screen and apply https://github.com/cabaletta/baritone/blob/master/SETUP.md#build-it-yourself to your own repo instead, using git and not the zip download. After you have done your changes commit them using git, push to your github repo and make a pull request. And please don't create junk pull requests like #2668.

I really should've just closed this as invalid with just my first sentence. Don't know why I've written almost a full essay for something that wasn't asked.