Andre Santana posts on Medium of a way he believes California could become a country without changes to the constitution. He refers to Article 4 Section 3 Clause 2:
“The Congress shall have power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States;”
The crux of the argument is that, since California was a territory before becoming a state, a simple act of Congress could perform the statehood process in reverse, reverting California back to a territory.
And there is a historical precedent, as in the cases of the Philippines and Cuba, to grant US territories their independence.