It's obviously best to complete homework, contest submissions and anything else with a deadline on time, but life doesn't always allow for that. Redditor lowbrassman2000 realised that when you need to make alterations after the fact, you should send a Dropbox link rather than the file itself.
As he explains:
Dropbox sends a link to the file on the server. The file can be altered or updated and the link is still the same. This allows for last minute changes and edits, assuming your professor hasn't saved a local copy of the file. ;)
While he is referring to homework assignments, the trick works just as well with anything you have to submit. If you need to buy yourself a little more time, use Dropbox.