In theory, any state should be able to secede. A lot of states wouldn't have even signed onto the Constitution if they thought they could never excuse themselves. They thought they could secede- and even then they took a lot of convincing to sign.

So any state should technically be able to secede. But as we all know, when states actually tried it, things got bloody (see 1861-1865).

But yes, Texas actually has as a condition of the deal that they could secede at any time.

So in theory, if Texas was to secede, the federal government should let that happen . In theory.