Hello! I don't know about resetting a tree, but you can always start a new one (which might be even preferable, so you can always see your first work and maybe compare something).
To start a new tree, click on "My Account" (on the upper right side of your screen). You'll see a list of your family trees and you can create a new one by choosing "Create or import new family" on the bottom of the page. If you want to, you can also remove your old tree from this list - however, it won't really be "deleted", just hidden. As far as I am aware, it is not possible to delete trees entirely.
Hope this helps!