It may be that you changed the title of the scenario in the share box. It must have the same name as you previously named it. If you want to change its name, change it on the site first; then, change it in the share box to that name to update it.
Yes, Now that I think about it. I changed the name of the on the website.
There's no such error message in Greenfoot. What exact message are you getting?
I didn't get an error, it said upload instead of update. But I think i figured it out.