Incorrect land value entry in prior year has resulted in the carryover of an incorrect basis How can I correct the 4794 worksheet to enter the correct number? Thank you!

Any corrections to prior year returns can be made by amending the returns for those years, if the error has occurred in the last 3 tax years.  (Currently 2014-2016.)  You would start by amending 2014 and work your way forward.

Otherwise, you will need to file Form 3115 "Application for Change in Accounting Method."  Form 3115 can be found in TurboTax Deluxe, Premier ,and Home & Business download products. 

Kitty, you misunderstood the question. How do I correct a worksheet in TT?

Go back through the "Asset" that you sold and enter the correct land value.

The worksheet won't let me make any changes. Need to change the original basis entered many years ago and carried over by TT. How do I change the worksheet entry?

The software won't let you change the asset basis because it cannot be changed after it was set up without notifying the IRS, therefore the requirement for Form 3115 - or amendments if you are within the correct time frame.  If you were able to override the numbers carried forward by the software, I am certain that you would not be able to e-file the return.

@benchmarkrealty   What "worksheet" are you referring to?  Are you entering things in the Step-by-Step interview, or the Forms mode?  If you are using the Forms, are you using the "Asset Entry Wks"?

@TurboTaxKitty   Correcting Basis is not an accounting change that requires Form 3115.  It is a "mathematical or posting error" than can be corrected on the tax return (without Form 3115).