https://stackoverflow.com/a/5542964
A submodule is its own repo/work-area, with its own .git directory.
So, first commit/push your submodule's changes:
$ cd path/to/submodule
$ git add <stuff>
$ git commit -m "comment"
$ git push
Then, update your main project to track the updated version of the submodule:
$ cd /main/project
$ git add path/to/submodule
$ git commit -m "updated my submodule"
$ git push
