You should be working in a fork, see the following documentation
On the
master
branch
$ git fetch --all
$ git rebase upstream/master
$ git checkout -b reason-for-changes
$ composer install
In the root directory, you can run the test suite by running:
$ vendor/bin/phpunit
$ git add [files...]
$ git commit -m "DESCRIPTION OF CHANGES"
$ git push origin master
To contribute your changes back, simply perform a Pull Request against the master branch.