And the media assets used in the project are present on both systems?
In your 2015 version bring up the project and go to the menu File > Export > Final Cut Pro XML, and export the XML (you can dismiss translation warnings). Transfer the XML to the other computer.
On your 2014 machine, open a new project and go to the menu File > Import and navigate to the XML file and import it.
You may need to reconnect your media.
Note that if you used functionalities in PPro 2015 that do not exist or are different in PPro 2014, they will not make the transfer.
MtD