YES!Yes, oh yes, there is a way to update your submission. I'll walk you through it.
1. Login
2. Go to Missions Depot and find your mission
3. You'll see a modify icon like this
Click it.
4. Now you'll see the mission details window. From here you can replace the submitted file with a new one, replace or add pictures, change the summary text, change version number, ect. ect. ect.
5. Push the Update button
6. Make a comment to the mission telling that you've updated the mission and what's different in the new version.
It's highly recommended that you update your submissions rather than submit new ones everytime. In fact I encourage updating so much that you can do a post in the 'mission depot comments' -board about your updated mission and it won't get locked as off topic or advertising.