I want to convert solidworks 2016 to solidworks 2015?is it possible.
I don't think converting is possible. But if you have ServicePack 5 of 2015 then you should be able to at least open a 2016 model in 2015...will be limited in what you can do with it but you can at least view the file...don't remember if it lets you save out as a step file or not while in 2015. Other options would be find a program that can open a SW file and save out as a STP file to reopen in SW2015 or go back to model source and try to get them to save out as a STP so can be imported into SW2015.
Did you tried to open the file with a text editor and see if the version is mentioned in the text?
This is not possible. Solidworks 2015 disabled this feature.
The only way is to save on step format (ap 204 or 209 if you can).