How do I change a stock's basis

the basis was incorrect when it was transferred into my current brokerage account over 10 years ago

Have you sold it?

No, but I intend to sell it this tax year.  Don't know if the brokerage firm (Fidelity) could/would alter or if there was another approach via the tax return filing.  Thanks.

You can change it on your tax return when you enter the sale.  The broker might not report the cost basis on the 1099B.