If the only change you are making only affects California then you do not need to amend the Federal return. If you change the amount of tax you paid to California in 2025 (such as withholding), then you should amend both returns if it will affect a deduction on the Federal return for taxes paid to CA.
If you need to amend only your state return, please see this help article for instructions for both TurboTax Online and TurboTax Desktop. You won't need Form 1040-X if you are only amending California.
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