You may not be able to do this in a desktop version of TurboTax. But, you should be able to do it in TurboTax Online.
Go to a Capital Gains Summary screen by either clicking on the Find icon at the top and typing "Capital Gains" (see 1st screenshot). Or by clicking on "Income" in the left-side vertical nav and then "Investments" > "Capital Gains Profile".
Once on the Capital Gains Profile screen, click "Continue" button. This should take you to the Summary screen. Click edit and enter the new amount manually.
I've provided screenshots below for your reference. Hope this helps.
