The Tenth Amendment to the Constitution of the United States reads, 'The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.'
No Article nor Amendment of the Constitution delegates authority over secession to the United States, and thus that power is retained by the several States, or to the people thereof. Thus it's pretty obvious that member states are free to leave any time they choose; it's the only reading of the supreme law of the land which makes any sense.