If you notice any mistakes and don't want to fix them yourself, open an issue on the GitHub page of the book. If you are reading this book with GitBook, you can also create a quick inline comment by clicking the "+" button for the paragraph.
Download and install the Atom editor (or your favorite editors: notepad++, vim, etc.)
Open Atom
Select "Open Folder" and select the folder where you cloned your fork
Edit the book with the help of the Markdown Preview package (Cmd/Ctrl-Shift-M)
Save files
Commit and push to your remote repo with your favorite git client (GitHub Desktop, Git BASH, SourceTree, etc.)
Make a pull request
Enhancing your learning process
Making contributions while you are reading is a good way to learn faster. If you have a hard time understanding something, try to reword it and make a pull request for other readers.
You can also help fixing issues. (Protip for university students: a good GitHub profile is more valuable than a diploma in the job market.)