To convert files from SoliddWorks 2013 to SolidWoks 2014, simply open these files with SolidWorks 2014. At the time of opening, SW specifies that these are files of an earlier version. They must then be saved and they are transformed into the 2014 version. The quickest way is to open the general assembly and then save, this transforms all the parts at once.
On the other hand, you can't do the opposite, i.e. have SW2014 files that you transform into SW2013. When you open an SW2014 file with a version of SolidWorks 2013, it specifies that it is a "future" version and you cannot work on it. it is irreversible.