Hi @uja2223,
If your institution is like ours, which I'm guessing is the case based on your description, courses will essentially go into a read-only mode (for faculty and/or students) upon their completion. This can sometimes help in keeping a somewhat accurate record of work and activity within a specific course, which can be beneficial in cases of grade disputes or other issues that could arise in the future. Unfortunately, it does mean that modifications to content need to be made somewhere other than that course. We allow faculty to request what we call a "work in progress" site for a course, which can be modified at any time and then imported into the next official course shell when it becomes available. Maybe this would work for you if your Canvas administrators would allow it.
-Chris