You would start the TT amended return process - that tells TT to "freeze" the original amounts so it has the starting $$ - then make your correction. Correct the state withholding amount wherever it was originally entered - if it's the state withholding on a W-2 then edit that W-2 and correct the number and the change will propagate through the Calif amended return normally.
If you itemized deductions on your federal return then that could change your federal taxable income and you should file that amended return as well.
Here is the FAQ on the TurboTax Support Site on amending a tax return:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
