If you tell the assistant "delete everything" or "let's start over," nothing gets deleted: by design, there's no button that wipes out your work without warning you first. Instead, the assistant walks you through the two real ways to start from scratch.
Option 1: a brand new website
Click "+ New website," at the top. Your current project stays exactly as it is, saved, and you start a completely clean one. This is the option when you want to try a different idea without touching what you already have, or when you run more than one business and need a site for each.
Option 2: redo this same website
If what you want is to leave this website as if you'd just started, without opening a separate project, tell the assistant about your business again —name, city, what you do, the style you're after— and finish with "create it again with this." The website gets regenerated entirely from what you just told it, counting as a regular generation that uses credits from your plan.
Nothing is ever lost
Whichever option you pick, earlier versions don't disappear: your history keeps every one of them, with its date, and you can go back to any of them whenever you want. Starting over is never destructive here.
Which one should you pick? If you're not sure you'll want to keep the current version around, use "+ New website": keeping both costs you nothing, and you decide later. If you're sure you want to replace this one, redoing it is more direct.
