devstack pulls it's openstack software stack from github. the git repot's it installs from are located in /opt/stack.
What you may want to do is fork openstack as well as the repot of the openstack projects you wish to modify, then make your devstack deployment deploy not from the openstack repot, but your own forked repot.
you can do this by modifying the stack.sh script ( I believe ). devstack.org has a line by line explanation of the entire script on their site and that can point you in the right direction.
http://devstack.org/stack.sh.html read this.
once you've deployed using your own git repository you can of course edit and commit back to it. even push to it.
then any other devstack deployment you have can also be setup to pull from your repository instead of the public openstack repos.
of course rebasing later against openstack will be increasingly difficult as openstack developement moves at a fairly brisk pace.
If the modification you are making is one you want to commit back to the open source project check out this site:
http://wiki.openstack.org/HowToContribute
Basically openstack has a commit review and continuous integration environment that is based off of gerrit and jenkins. This is the method by which commits back to the open source repository are gate tested and manually reviewed by other developers before being merged.
If you are intending to deploy this for production use, I recommend against deploying from devstack. This is not the proper way to do that.
