Sewing Machine Question?

Ok, so i just got a brand new sewing machine 2 day and when i opened it there already was a peace of fabric on it ready to sew.... but now i want to change the fabric do i do that? When i was in girls scouts we were tought how 2 tttttttttt i forget... Do i pull the fabric then cut the 2 strings and tye a little knot in it is that safe?