You don't have to delete, fix, and re-post your scenario. Just follow these steps:
1. Open Steam.
2. Click on "Browse the Workshop".
3. In the "Your Workshop Files" panel (on the right), hover over "Your Files" and select "Files you've posted".
4. Select the scenario you want to modify and wait for it to open (if a message claims that Steam isn't available, shut it down and start over or, if that doesn't work, wait until later and try again.)
5. Below the scenario's picture, on the right, you'll see "Owner Controls", where you can edit the title and description, add or edit images and videos, and so on.
Hope that this helps. If you feel the need to edit the scenario itself, you will have to delete it from the workshop (there's a button for that, too) and then upload it again after making your changes. In that case, I'd suggest changing the title slightly by adding something like "fixed", "edited", "new version", etc.
