Of course they can.
States created fedgov. They can leave.
Doesnt mean fedgov will accept it, but they sure the hell can.
Straight from the NH Constitution:
[Art.] 10. [Right of Revolution.] Government being instituted for the common benefit, protection, and security, of the whole community, and not for the private interest or emolument of any one man, family, or class of men; therefore, whenever the ends of government are perverted, and public liberty manifestly endangered, and all other means of redress are ineffectual, the people may, and of right ought to reform the old, or establish a new government. The doctrine of nonresistance against arbitrary power, and oppression, is absurd, slavish, and destructive of the good and happiness of mankind.