So you have already Submitted and Finalized your proposal but you now wish to undo it. As long as the project is still “Open” (i.e., still accepting bids) you will be able to un-submit your proposal.
Log into your Bonfire profile using your email and password, click on the appropriate project, scroll to the bottom of the page and click on the View Receipt button.
On the subsequent screen, scroll to the very bottom and you should see a "Click here to un-submit your submission" link:
You will get a confirmation pop-up. Once you acknowledge that, it will return you to the Complete Your Submission screen where you can make changes accordingly and Submit and Finalize whenever you are ready.