Open an issue regarding an improvement you thought of, or a bug you noticed, or ask to be assigned to an existing one.
If the issue is confirmed, fork the repository, do the changes on a separate branch and make a Pull Request. After review and acceptance, the PR is merged and closed. Please don't forget to add unit tests for all the changes that you make.
Also, please make sure your PR is small: we usually reject any PR which is bigger than 5 changed files, including the tests.
Make sure the maven build
$ mvn clean install -Pcheckstyle,itcases
passes before making a PR.