Turtles never come out of their shells. A turtle shell grows with the turtle, so there is no reason for a turtle to swap shells. If a turtle shell receives any damage, it can repair itself the same way as any other living part of the turtle.
Turtle shells consist of bone. These bones include as many as 60 rib and back bones that grow together to form the upper shell, or carapace. This connects to the clavicles in the lower shell, which is known as the plastron. The entire shell is covered with scutes, which, like human fingernails, are made of keratin. Scutes provide a protective coating for the shell. They are shed and replaced by larger scutes as the turtle grows.