Nothing in the Constitution prohibited secession; nothing in the Constitution *authorized* secession, either.
https://en.m.wikipedia.org/wiki/Secession_in_the_United_States#Dubious_legality_of_unilateral_secession
To this day, the Constitution is silent upon that question. But in formal terms, the 13 colonies and every state that joined thereafter gave up their absolute sovereignty by ratifying the Constitution in order to enter the Union and the federal structure it entails.
And in practical terms, the Civil War settled the question of secession. At least, in the unilateral manner the Confederacy pursued secession in and under the particular circumstances of the time.
If ever in the future an American state or group of states were to decide to go its own way, the movement for secession would be better served trying to seek a diplomatic consensus from the remaining states for a peaceful split.
It stands to reason there would need to be a Constitutional convention to authorize the proposed secession, since no procedure for secession currently exists. Secession cannot currently be authorized by the Executive Branch, the Legislative Branch, the Judicial Branch, or even all three branches acting together.
So: A tough order, but a Constitutional convention would be one way to leave the Union without being traitorous.