How do I convert a second home to investment property?

You would begin in the Rental section of the Premier Edition.  You will encounter screens that allow you to convert from personal to business. Please see screen shots below:

I want to convert residential single family income property to my second home

What form allows this ?

Don't enter this in the forms mode or attempt to enter it. Instead go through step by step by doing the following. if entered correctly, the information will flow to the correct forms in Turbo Tax

1. Select the federal TAXES  tab, then go to wages and income.
2.  Scroll to Rentals, Royalties and Farms
3.  Select Rental Properties and Royalties (Sch E) Select start
4. Then you will go through a series of questions regarding your rental property. 
5. When you arrive to a screen that looks like the screenshot I have included below, indicate you converted it from a rental to personal use.