Submodule repositories stay in a detached HEAD state pointing to a specific commit. Changing that commit simply involves checking out a different tag or commit then adding the change to the parent repository.
$ cd submodule
$ git checkout v2.0
Previous HEAD position was 5c1277e... bumped version to 2.0.5
HEAD is now at f0a0036... version 2.0
git-status
on the parent repository will now report a dirty tree:
# On branch dev [...]
#
# modified: submodule (new commits)
Add the submodule directory and commit to store the new pointer.