If you created your family tree in Family Tree Builder, all changes and updates to the tree should be made there. When you finish, republish the tree to your family site.
If you created your family tree directly online or uploaded a gedcom, then you can make changes and updates to the tree from your site. If you move the mouse over a person card in the tree, you'll see a little pencil icon. This is the Edit Details window. Click on the little pencil to add/edit information for someone.