Can I transfer 2016 tax data from mac to a windows 10 laptop for 2017 taxes? How? I need to know before buying.

Assuming you completed your 2016 return with the CD version you installed on your computer, search for the file ending with a .tax2016 filename extension. Then after you install the program on the windows computer and *before* you start the program the first, time, place that .tax2016 file in the documents/turbotax directory on the windows computer.

What f I didn’t use CD but downloaded the program

If you had to physically install the program on your computer, be it from a CD or from an installation file you downloaded, and if you can start the program on your computer while your computer is physically disconnected from the Internet, then you are using the Cd/Desktop version of the program.