One of the many great things about the submission process is that Bonfire automatically saves your progress on the fly as you work! In other words, you can start your submission and resume it whenever you'd like without having to submit/finalize your work until you are ready to.
The next to time you log in, simply go back to the Project you were submitting to by clicking on the corresponding View Opportunity button.
Then, scroll to the Submission section near the bottom of the page. Here you will see your Work-in-Progress submission(s). Click the Continue Submission button to resume exactly where you last left off!
Alternatively, you can click on Submissions in the toolbar at the top of the page of your portal.
From there, you can navigate to your Work-in-Progress Submissions and select View from the Actions drop-down menu to continue the submission you previously started.
If you've already finalized and submitted and need to modify your submission, see Can I un-submit my bid once it has been submitted and finalized?