Saving as draft when submitting an assignment
When a student attaches a file to an assignment, they created an assignment submission. Most assignments allow the submission to be saved as a draft. In Blackboard Learn, the upper right hand corner of the window shows the submit button as well as the save draft button. If you intend to put a different document into the submission before you submit, then you MUST attach the second document to the assignment before you remove the first document. Once the second document is attached, then you may mark the first document for removal.
If you do not attach a second document before removing the first, then your assignment will be locked and you can no longer add new attachments to your assignment.
These screen captures show the steps involved in making a submission, adding a second document to that submission and finally marking the first to be removed.
DO NOT MARK A DOCUMENT FOR REMOVAL UNTIL YOU HAVE ATTACHED ANOTHER DOCUMENT.
This is the proper way.