Do you mean to amend to Married filing Separate? It's too late to change to MFS if you already filed Joint. But here is a CA page on eligibility. Scroll down to see MFS and amounts.
https://www.ftb.ca.gov/about-ftb/newsroom/golden-state-stimulus/help.html
If you file MFS you may end up owing more than you would get for the Stimulus. So it may not be worth it.
