Comment: It's in the Declaration

The right to secede was curiously left out of the Constitution. It's an interesting question, and if I'm not mistaken there is some back story to this. During the conventions this may have been brought up, but I'd have to research that.

Also, some of the State Constitutions may list the right. Here is Pennsylvania's, as an example:

"Section 2. Political Powers
All power is inherent in the people, and all free governments are founded on their authority and instituted for their peace, safety, and happiness. For the advancement of these ends they have at all times an inalienable and indefeasible RIGHT to ALTER, REFORM or ABOLISH their government in such manner as they may think proper."

And here is our founder's declaration:
"But when a long train of abuses and usurpations pursuing invariably the same object, evinces a design to reduce them under absolute despotism; it is their right, it is their duty to throw off such government and to provide new guards for their future security."

Not just a right, but a DUTY! Hah, imagine that, we are duty bound to throw off tyranny. This is the only duty that is ever mentioned in the any of our founding documents. Notice the duty isn't to don a gov't issued uniform and kill foreigners for "merican freedumb", or to cast a ballot for 1 of 2 millionaire strangers with law degrees.