Legal Question in Civil Litigation in California

No. The Constitution describes how states can be added but says nothing about states leaving the union. That means there is currently no way for a state to leave. If California's government and/or voters tried to do this unilaterally, the federal government would not recognize or accept their actions.

The Constitution could be amended to add a method for states to secede. It is hard to imagine such an amendment getting more than token support, much less actually passing. Remember that passage would require two-thirds support in both the House and the Senate, plus ratification by the legislatures of three-quarters of the states. That isn't going to happen anytime soon.

Another way to amend the Constitution is via a constitutional convention. Such a convention can only be held when two-thirds of the states demand it. It is unlikely that a convention would approve an amendment allowing states to secede. But even if it did, the proposed changes would have to be approved by the legislatures of three-quarters of the states.

I thought that issue was resolved by the Civil War. Do you have some more specific legal question, such as whether a state law is preempted by federal law?