Well, if the people of California all wanted to leave, they could elect a legislature and a Congressional delegation that is pro-secession. This is farther than China will allow Hong Kong.
After this, the California Congressional delegation could introduce a California secession bill. This bill would be debated in Congress.
If they were really serious, the Californians could support a constitutional amendment specifically allowing secession sort of like Article 50 of the EU.
But there are other example of people having this right. The people of Scotland narrowly voted to remain in the UK (but could have left). The people of Northern Ireland have the option to leave the UK. The UK chose to leave the EU. Puerto Rico could probably leave the US (who currently rules them) if they wanted to.
After this, the California Congressional delegation could introduce a California secession bill. This bill would be debated in Congress.
If they were really serious, the Californians could support a constitutional amendment specifically allowing secession sort of like Article 50 of the EU.
But there are other example of people having this right. The people of Scotland narrowly voted to remain in the UK (but could have left). The people of Northern Ireland have the option to leave the UK. The UK chose to leave the EU. Puerto Rico could probably leave the US (who currently rules them) if they wanted to.