I would advice against merging your personal footprints/symbols locally with the oficial library.
The next update might wipe your personal stuff. It is much better to have your stuff in your own libraries.
Also footprints libs normally do not exist locally. For them to exist locally you need to use the library wizard and use the save local copy option. (details for local setup: Using Github libraries offline)
If you still want to do this go with what @keruseykaryu explained.
Or do you mean you want to share symbols/footprints you created with the community?
If so: first read the library convention to make sure your stuff is designed such that it can be used.
Also have a look on the new library convention. It will be published soon.
@SchrodingersGat also made a FAQ that is helpful
Now to the how to:
You will need a github account.
Log in to the github site and fork the repositry in question.
(There is one repository per footprint library and one single repo for all symbols.)
The symbol repo: https://github.com/KiCad/kicad-library
For the footprint repos just look in your fp-lib-table. There you can find the correct url. (or use the search function or go to https://github.com/KiCad and scroll until you find the repo you need.)
After you forked the repo, clone it to your local machine.
Use the clone or download button on github to get the correct url.
Make a new branch with a meaningfull name.
Make your changes and add/commit/push them to your fork.
After this is complete go to the website of the original repo and create a pull request.
In the description of the pull request add the information necessary (a link to the datasheet, a screenshot that shows your work and a short description of what you changed.)
It normally takes a few days until a library manager gets back to you with either suggestions for improvements or with a thanks for your contribution and subsequent merge.
To help with checking the quality of the submissions some automatic tests are run on each pull request. Make sure you read the results of this automated test system and fix the issues found by it. (You can also run test scripts locally. They can be found under: https://github.com/KiCad/kicad-library-utils)
There is also a detailed how to on the wiki of the packages3d github page: https://github.com/KiCad/packages3D/wiki/Contributing
The one on the library github page also works, but it does not have that many git commands explained.