Can you please clarify for which section in your return you need to make the adjustment? Is it a for an investment property, main home etc...?
After I have completed the federal and state taxes, Turbotax took me to the next tab, Review. Here is where I was asked to Fix errors. Then, a page displayed with a statement that I needed to enter the cost/adjusted basis for the Noncash contribution. I wanted to change the amount I entered here. I returned to to the Review tab but to no avail. I was unable to bring back this page where I could enter a new amount.
To adjust your cost basis in a charitable contribution in TurboTax Online: