how to open a file solidworks 2013 on solidworks 2012?

You can only open if you have SolidWorks 2012 SP5, but you don't have any feature or histori.

You can not open the parent version file in the previous software version.
( if not sp5 last versiyon ! ) Generally, file formats with different extensions such as iges, sat, etc. are accepted. In such a case you can not change the 3d drawing, it is difficult.

you can convert it to step file...then you can open your object with solidworks 2012. at this moment you don't have feature in history. to get the feature you have to go t o insert> feature works> recognize feature....then select the features you want and bring it
I think it will help you to get the features in history.

Save As Neutral Format: If possible, open the SolidWorks 2013 file in SolidWorks 2013 and save it in a neutral format that is compatible with SolidWorks 2012. Common neutral formats include STEP (.stp) and IGES (.igs). To do this:

Open the SolidWorks 2013 file.
Go to File > Save As.
Choose either STEP or IGES as the file format.
Save the file with a new name.

There is a software developed called BackWorks. You can find more info in the link
http://backworks.info/

You can only open future version files in the SP5 of the previous of SolidWorks. For example you can open SolidWorks 2014 files in SolidWorks 2013 SP5.