Hello Vortrog and all,
I don't want to sound rude, but you can update your own missions. This is actually a feature that's been in use for quite some time now. Maybe we'll need a quick how-to on the updating process.
Here's what you do.
After logging in on the site or forums (if not 'always logged in' selected) go to the missions depot. Search for the mission you want to update. Note that you can update only your own missions.
Look at the right hand side of your mission overview field. If you're logged in and the owner of the mission there should be a small button like this
Click on the 'modify'-button and a new window will open.
Scroll down the window and you'll see where the file can be replaced. It looks like this.
Check the 'replace file'-box and a text field under it becomes visible. Click on 'browse' ('selaa' in the picture above). Find and select your new mission.zip from your harddrive and click 'ok'.
Then click the 'Replace'-button and the old file should be replace with the new one you just selected.
Easy as pie. Mmmmm..
* Artak goes get some pie