Article 50 of the Treaty on European Union (TEU) allows a country to leave the EU if it wants to. A country has to notify the European Council of its intention to depart. Next, the Council provides instructions on how to exit the EU.
With the consent of the European Parliament, the European Council makes decisions about the withdrawal deal. A nation that has left the EU may reapply to enter. It would have to go through the same procedure as if it were a new member nation.