As Texas Roger says, only one tax year is available online at any one time. You will need to acquire a desktop version of TurboTax to amend your income tax return and the link Texas Roger provided should guide you.
But, forgetting about the 1099-B correction issue, did you actually report the transaction correctly in the first place? This was a fairly difficult cash plus stock merger that required some "special" income tax accounting. And deals like this get tougher as the number of lots involved goes up. It would be interesting to see some details being reported by the 1099-B, (personal information removed, of course), to see if the "basis correction" really pertains to the special accounting required here.
This answer
https://ttlc.intuit.com/questions/3146739-when-do-i-need-to-enter-a-cost-basis-on-brokerage-transact...
provides an overview of the account for this merger.
Tom Young
