If you want to improve on the design and contribute you'll first need to import the project files into your workspace, I used EasyEDA for all my projects, but if you're using something else I believe by now these projects can be imported from a couple of platforms, I use EasyEDA because it's simpler to me, I can just order the PCB directly from the application and it just works (I am by no means sponsored or supported from the company in any way, just sharing what works for me).
You can fork this repository, work on your own improvements, adjustments and hopefuly you would submit a PR for your changes to be merged into this main repository, thus sharing with everyone your updates as well, after of course sharing review of the changes and testing.
You can use EasyEDA to open all project files and import into your own workspace by importing the files located inside the "EasyEDA", from the application you can make your adjustments, and update the schematics PDF files and Gerber files where available and update the repository with your changes.