Journal assignments are in most cases not bound to a specific deadline, unless the course instructor requests such a, and can be edited afterwards. To edit your journal assignment, click on "Journal" in the navigation bar in a specific course.
Under "My Journal" you have an overview of your journal entries. If you choose a specific entry you have the option to edit or delete your entry. Please keep in mind, that it is not appropriate to change your journal entry if someone has already commented on it. In this case it is better to answer the comment.