I received an answer about using straight line rather than the default MACRS method of 150DB for 15 years. I want to use SL for 39 years. How can I override the default?

What kind of property?

office building

A building is a 39 year property ... not sure how you can get the program to do anything but that.  Try deleting the asset and enter it again to pick the correct asset class.

When you use Turbotax Online version, there is no way to enter directly on forms and there is no override function.