There is no mention of, or stated process for, secession of states in the Constitution. In Texas v. White:
"Chase, [Chief Justice], ruled in favor of Texas on the ground that the Confederate state government in Texas had no legal existence on the basis that the secession of Texas from the United States was illegal. The critical finding underpinning the ruling that Texas could not secede from the United States was that, following its admission to the United States in 1845, Texas had become part of "an indestructible Union, composed of indestructible states." In practical terms, this meant that Texas has never seceded from the United States."
https://en.wikipedia.org/wiki/Secession_in_the_United_States...