I end up there by searching how to update specific submodule only, which means for me, updating a submodule to the ref pointed by its super-repo.
Which is not the question nor the answer but only the title.
So in a hope of helping some others like me the answer to the question title is :
git submodule update <specific path to submodule>
which will put this submodule in the state of the ref commited in the super-repo.