Just work through the asset in the assets/depreciation section on your 2021 return. When you click the EDIT button next to an asset, the very first screen will show you the "cost" and "cost of land" originally entered in the year you acquired the property, or the year you converted the property to rental; whichever occurred last.
Under no circumstances and with no exceptions should you ever change the cost basis of an asset. Doing so will completely skew the depreciation history, and the current year's depreciation taken will be wrong. There are no exceptions.
While it may not happen quickly, changing the cost basis of an asset "will" raise flags, and you'll get a nastygram from the IRS generally 24-36 months after the filed return is accepted and processed.
