Moodle allows you to upload a draft of your assignment to have something on file by the deadline even if you are not yet completely finished. Draft status allows you to come back to the assignment and make changes. To change your submission, delete your draft and upload your new version. Once your paper is finished, make sure you complete the last step for submitting it, which changes the status of what you uploaded from "draft" to "submitted."  Once submitted, this is the version of the paper that will be graded. If you skip this step, your draft will be graded instead, regardless of the state it is in.