After you submit on codePost, you can always resubmit, so long as the due date of your assignment hasn't passed.
To resubmit, just hit "View assignment" and then make sure you're on the "Submit" tab. From here, you can re-submit to overwrite your past submission.
Note that your instructor may limit the number of times you can receive automated test results. If you use up your test runs, you can still re-submit, but you won't receive any autograder output when you do.