Article 3, Section 3, Clause 1. Read it!
It becomes more interesting when you consider the 1869 SCOTUS case of "Texas v. White", wherein the majority opinion declared that a state could not leave the Union except through "revolution or through consent of the States."
The question then becomes, however, assuming the factual claims within the majority opinion — namely, that Texas never actually seceded — have any veracity, then what would the ordinance of secession (approved by the Texas legislature on 2/01/1861, and approved by a statewide referendum on 2/23/1861 with a more than three-fourths majority) have constituted if not a revolutionary act in and of itself?