how to save Solidworks 2016 files as Solidworks 2015 files?
I want save solidworks 2016 part file in solidworks 2015 as like autocad ?
You can't do it.
If you have the latest service pack for 2015 installed (5?), 2015 might open 2016 files, but I've had mixed results.
I don't think it would be worth the effort, but you could use the Feature Recognition tool in SolidWorks when importing a "dumb solid" into SW2015.
Users have been asking for backwards compatibility for two decades. The closest we have is using the latest service pack in the previous year's release. Beyond that, you'll get the "future version" error.