You can edit the existing content of the assignment by enabling the edit mode. Please follow the below steps to edit the contents:
Go to the Courses section.
Select the course.
Go to the Edit tab
Click on the assignment name to open and edit the content.
For editing the contents, check out the editing content section of this document for more details.
When changes are ready, publish the assignment, either from the Publish button that will show or from the Education->Publish Assignment menu item in the project.
Check out this article for publishing the assignment after editing the content.