That is NOT an option at this stage. When you create a new AdBoard we generate a complete website using that SiteSlug as the directory name, so it is an integral part of your website's address.
If you change that, your website will stop working and anyone sending traffic to that link will get a 404 error... which hurts us all in the search engines.
You can delete the AdBoard so you can start over, but be aware, if you have ANY Advertisers at all, then their Ads will be lost, so that option should be a last resort… ideally only done after you have contacted the Advertisers directly to warn them.