Information on the basic method of working with testcases can be found at the Ubuntu QA Team Manual Testcases page.
We are only concerned with a specific set of tasks: grabbing the branch, making edits and then pushing the changes we need to the main branch. We have people in the Testcase Admin team for the LP Testcases, in addition any member of our Release Team can edit the tracker, this helps ensure that changes are moved through to the trackers quickly.
To edit a testcase:
Report the required change as a bug against the testcase project
Assign yourself to the bug
Create a local branch: bzr branch lp:ubuntu-manual-tests
Make changes to the testcase in your local branch
Commit the changes: bzr commit -m "Fix LP bug #BUGNO."
Push to a personal remote branch: bzr push lp:~username/ubuntu-manual-tests/bug-BUGNO
Once pushed to a personal remote branch, propose the change for merging - Submitting merge proposals.
Respond to any requests for changes when asked by the Testcase Admins in order to get the required change through in a timely manner.