You cannot modify Assignment Content once the assignment is created. However, you can delete an assignment, which moves all the content in the Assignment back to the Unassigned Content area. You can then create new Assignment(s) with that content. Any work that students did on that content will be saved and will be visible to students again once you create a new Assignment with that content.
To delete an Assignment, click on the trash can icon to the right of the name of the Assignment.
Note: If a student is in a module and actively submitting answers when the Assignment is deleted, any answers submitted after the deletion will be LOST FOREVER. Thus, Assignments should only be deleted BEFORE or AFTER there is any likelihood that students will be actively submitting answers.