Hello! Sadly, I believe it is not possible to delete people on a family tree in batches - you'll have to delete each sibling seperately. To do this, click on the person you want to delete, select "More Options" in the sidebar on the left, and then "Delete [person's name]".
To start a new family tree, you'll have to go to "My Account" - there is a button for this on the upper right side of your screen between "Save" and "Logout". When you are on "My Account", you'll see a list of all your families, and create a new family via the button "Create or import new family". Behind every family listed on "My Account", you'll also have the ability to change a family tree's name via the "Rename" button.
If you want to import (parts of) a former tree, you can do so by downloading it via the "Save" button on the lower side of your screen when editing it and selecting GEDCOM format. If your problem is that the wrong person on your tree is shown as "Founder"/"Me", you can change this by clicking on "Tools" (also lower side of screen) and then "Change default person".
Hope this helps!
