From SW2012 SP 5 you'll have the option to do this but for now there is no direct option. The only way is to get the files converted in iges, step, etc. format and then open in lower versions.
BTW if you've used the search option before putting up the question here, you would have got the answer.
Deepak where did you here about service pack 5 and the save to older version?
can you put up a link so we can read it?
Deepak did not give the complete story.
At SWW2012 they announced that SW2012 SP5 will be able to open SW2013 files. I am assuming going forward that the last SP of a SW release year will be able to open the next years version of SolidWorks. I would not expect to see anymore backwards compatibility then that.
Nothing was mentioned about being able to do a Save As to a previous version of SolidWorks. Just that SW2012 SP5 wil be able to open SW2013 files.
Just a bit of backwards compatitbilty to make upgrading to the next version of SolidWorks easier for companies.
Like it has been mentioned short of getting the file in the parasolid, iges or step file or dxf for a drawing from the future
version there is no way.
Understand if you do go this route the iges, parasolid and step files will come in as dumb solids menaing there wont
be any dimensions, features or constraints attached to them.