The user interface is somewhat different in the Online and Desktop versions, but both offer the user a way to edit a specific transaction. Click on the pencil, rather than the check box.
In the Desktop versions, from the Income Summary, you can open the investments, Stock transactions category, click edit, and then click on the pencil icon next to the specific transaction you would like to edit.
In TurboTax Online, open Wages & Income, scroll down to Investments and click Add/Edit. Once you have entered all of the transactions, you can click the pencil icon next to the specific transaction you would like to edit.
See screenshot below (the "pencil" is highlighted):

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