You probably don’t need to update the Core Library by hand, that will be updated with every update of silverbullet, if it needs to.
You can delete index.md and create yours, I believe we all do that and customized it to our needs.
Have a look at this post: What does your Index page look like?
As for Journal, nothing better than to point you to the official page which talks about it: Library/Journal
It helps creating a daily/weekly page