If your instructor has set up the assignment to allow you to resubmit an assignment, you will be able to do so until the due date. Follow the instructions for the initial submission to resubmit the assignment. Note: You can only resubmit three times to receive an immediate similarity report. After that, you will have to wait 24 hours to receive the similarity report.
If your instructor has not set up the assignment to permit resubmission, then you will have to contact your instructor directly. They will have to manually delete the previous submission before you can resubmit the assignment.