How do I get the land portion of a previous year's investment property purchase, into turbo tax?

I purchased income property 20 years ago and did not enter the land value in my taxes.  I only entered the building.  Now, in 2018, I have sold the property and need to apply the sale against the entire property, including the land.  I don't know how to get the land into turbo tax to do this. Can someone help?